A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | Z
| Term | Definition |
|---|---|
| A (index) | |
| Aboiteaux | 1) Dike and drainage structure protecting marshlands. 2) Outlet structure with flap gate which permits outflow of fresh water, but prevents inflow of tidal salt water. (No longer in common use.) |
| Acid mine drainage | Water draining from areas that have been mined for coal or other mineral ores. The drainage water is acidic, sometimes having a pH less than 2.0, because of its contact with sulfur-bearing material. |
| Acid rain | Precipitation that has a low pH (less than 5.6, which is normal for natural precipitation). The precipitation becomes acidic when moisture in the air reacts with sulfur and nitrogen pollutants in the atmosphere. Acid rain has a harmful effect on some plants, aquatic organisms, soils, and buildings. |
| Adapter | A coupler fabricated for attachment of hose, threaded pipe, or other devices to irrigation pipe, or for connecting irrigation pipes of different sizes. |
| Advance time | Time required for a given stream of irrigation water to move from the upper end of a field to the lower ends. |
| Aerobic decomposition | The decay organic matter by microorganisms In the presence of oxygen. |
| Air counterfow | Upward movement of air through soil during downward flow (percolation) of water. |
| Air drainage | (Soil) Renewal of soil air by diffusion. |
| Air lift pump | An apparatus for lifting water by using the buoyancy of injected air. |
| Alfalfa valve | An outlet valve attached to the top of a pipeline riser with an opening equal in diameter to the inside diameter of the riser pipe and an adjustable lid or cover to control water flow. A ring around the outside of the valve frame provides a seat and seal for a portable hydrant. |
| Algae | Simple rootless plants that grow in sunlit waters in relative proportion to the amounts of nutrients available. They can affect water quality adversely by lowering the dissolved oxygen in the water. They are food for fish and small aquatic animals. |
| Algae blooms | Rapid growth of algae on the surface of lakes, streams, or ponds; stimulated by nutrient enrichment. |
| Algal bloom | Large, visible, masses of algae that develop in bodies of water during warm weather. |
| Algicide | Any substance that will kill or control algal growth. |
| Alkali | Any strongly basic substance of hydroxide and carbonate, such as soda, potash, etc., that is soluble in water and increases the pH of a solution. |
| Alkali soil | Soil containing sufficient exchangeable sodium to interfere with water penetration and the growth of most crops. The exchangeable-sodium content is greater than 15%. (Preferred term is Saline-sodic soil.) |
| Alternate set irrigation | A method of managing irrigation whereby, at every other irrigation, alternate furrows are irrigated, or sprinklers are placed midway between their locations during the previous irrigation. |
| Alternate side irrigation | The practice of furrow irrigating one side of a crop row (for row crops or orchards) and then, at about half the irrigation time, irrigating the other side. |
| Anaerobic decomposition | The decay of organic matter by microorganisms in the absence of oxygen. |
| Anisotropic soils | Soils not having the same physical properties when the direction of measurement is changed. Commonly used in reference to permeability changes with direction of measurement. |
| Apparent flow velocity | (Porous media flow) Flow rate of water passing through a unit cross section of porous media. |
| Application efficiency | The ratio of the average depth of irrigation water infiltrated and stated in the root zone to the average depth of irrigation water applied, expressed as a percent |
| Application rate | Rate that water is applied to a given area. Usually expressed in units of depth per time. |
| Apron | Floor or lining extending downstream from a hydraulic structure to protect the structure from erosion and scour. |
| Aquiclude | Underground geologic formation that neither yields nor allows the passage of an appreciable quantity of water, although it may be saturated with water itself. |
| Aquifer | A geologic formation that holds and yields useable amounts of water. Aquifers coo be classified as confined or unconfined. |
| Aquitard | Underground geologic formation that is slightly permeable and yields inappreciable amounts of water when compared to an aquifer. |
| Arched dam | Curved masonry or concrete dam, convex upstream, that depends on arch action for its stability. The load is transferred by the arch to canyon walls and bottom or other abutments. |
| Arid climates | Climate characterized by low rainfall and high evaporation potential. A region is usually considered as arid when precipitation averages less than 250 mm (10 in.) per yr. |
| Artesian aquifer | Aquifer that contains water under pressure as a result of hydrostatic head. For artesian conditions to exist, an aquifer must be overlain by a confining material or aquiclude and receive a supply of water. The free water surface stands at a higher elevation than the top confining layer. |
| Aspect | Compass direction that a slope faces in a water catchment area |
| Assimilative capacity | Natural ability of soil, air, or water to accept and/or degrade potential pollutants without harmful effects to the environment. |
| Atmosphere | The layer of gases surrounding the earth and composed of considerable amounts of nitrogen, hydrogen, and oxygen. |
| Atmospheric water | Water present in the atmosphere either as a solid (snow, hail), liquid (rain) or gas (fog, mist). |
| Auxiliary pump | Any pump designed to perform a service to the main pump or power unit, such as a cooling water pump or a standby pump to supplement the system for peaking requirements. |
| Auxiliary spillway | Dam spillway built to carry runoff in excess of that carried by the principal spillway. (preferred term is Emergency spillway.) |
| Available soil water | The portion of water in a soil that can be readily absorbed by plant roots. It is the amount of water released between in situ field capacity and the permanent wilting point. |
| Axial-flow pump | A rotary pump that develops head mostly by the propelling or lifting action of the vanes on water, commonly referred to as a propeller pump. |
| B (index) | |
| Backflow prevention device | Safety device which prevents the flow of water from the water distribution system back to the water source. |
| Backfurrow | A ridge formed by a plow when soil is moved together or on top of undisturbed soil. It is the opposite of a dead furrow. |
| Backslope | Land area on the downhill side of a terrace ridge or earth embankment. |
| Baffles | Vanes, guides, grids. grating. or similar devices placed in a conduit to regulate water flow, effect a more uniform distribution of velocities. reduce pressure losses, deflect or create mixing. |
| Bank storage | Water leaving a stream channel during rising stages of stream flow, most of which returns to stream flow during falling stages. (See Floodplain storage.) |
| Base area | 1) The cross-sectional area of the base of a cone penetrometer. 2) Contact area of a dam with original surface. |
| Base line | 1) A reference line from which measurements are taken. 2) A parallel of latitude for public land surveys. |
| Base saturation | Extent to which a material is saturated with exchangeable cations other than hydrogen, expressed as a percentage of the cation-exchange capacity. |
| Basic intake rate | Rate at which water percolates into soil after infiltration has decreased to a low and nearly constant value |
| Basin | 1) (Hydrology) Area drained by a river and its tributaries. 2) (Irrigation or recharge) Level plot or field, surrounded by dikes. which may be Hooded. |
| Basin irrigation | Irrigation by flooding areas of level land surrounded by dikes. Used interchangeably with level border irrigation, but usually refers to smaller areas. |
| Batter board | One of a series of horizontal boards set across or to one side of a trench line to indicate a desired elevation or reference grade from which trench bottom elevations are determined. |
| Bed load | Coarse sediment or material moving on or near the bottom of a flowing channel by rolling, sliding, or bouncing. |
| Bedding | : 1) A surface drainage method accomplished by plowing land to form a series of low narrow ridges separated by parallel dead furrows. The ridges are oriented in the direction of the greatest land slope (crowning or ridging). 2) Preparation of furrow irrigated row cropped field with wide, flattened ridges between furrows on which one or more crop rows are planted. 3) The process of laying a pipe or other conduit in a trench with the bottom shaped to the contour of the conduit or tamping earth around the conduit to form its bed. The manner of bedding may be specified to conform to the earth load and conduit strength. |
| Bedding angle | The acute angle of a V-groove in the bottom of a trench for support of pipe drains. |
| Bedding ditch | A dead furrow used as a surface drainage ditch in a bedding system. |
| Beds-in-basins | Ridges raised above the ponded water surface of a level basin, with channels between (dead level furrows or basin furrows). |
| Beehive | Dome-shaped grating placed on surface inlets to subsurface drains to exclude trash. (Preferred term is Trash rack.) |
| Bench flume | A water conducting channel built on constructed terraces along hillsides or around mountain slopes when the ground is too rough, steep, or rocky to permit an excavated canal. |
| Berm | Strip or area of land, usually level, between the edge of spoil bank and edge of a ditch or canal. |
| Best management practice, BMP | Structural, nonstructural, and managerial techniques recognized to be the most effective and practical means to reduce surface and ground-water contamination while still allowing the productive use of resources. |
| Bioaccumulation | A general term describing a process by which chemical substances are consumed and retained by organisms, either from the environment directly or by eating food containing the chemicals. |
| Biodegradation | Breaking down of natural or synthetic organic materials by microorganisms in soils, natural bodies of water, or wastewater treatment systems. |
| Biological diversity (biodiversity) | The variety of different species, the genetic variability of each species, and the variety of different ecosystems that they form. |
| Biomagnification (biological magnification) | A cumulative increase in the concentrations of a persistent substance in successively higher levels of the food chain. |
| Biota | Collectively, the plants, microorganisms, and animals of a certain area or region. |
| Black alkali soil | A soil with a high pH and chemistry dominated by sodium carbonate. Dissolved organic matter may be deposited on the soil surface as soil water evaporates. (Preferred term is Saline-sodic soil.) |
| Black water | Water containing liquid and solid human body waste generated through toilet usage |
| Blind drain | Type of drain consisting of an excavated trench, refilled with pervious materials such as coarse sand, gravel, or crushed stones, through whose voids water percolates and flows toward an outlet. (Also called a French drain.) |
| Blind inlet | Surface water Intel to a drain in which water enters by percolation rather than through open flow conduits. |
| Blinding | Material placed on top of and around a drain tile or conduit to improve the flow of water to the drain and to prevent displacement during backfilling of the trench. |
| Blow-off valve | Controlled pipeline outlet used to discharge water. |
| Blowout | A rupture in a pipe drain usually attributed to hydraulic pressure from within the line and resulting in displacement of pipe and in washout of supporting and covering earth materials. Also break troughs or ruptures in embankments caused by piping. |
| Bog | A type of wetland that accumulates appreciable peat deposits. They depend primarily on precipitation for their water source, and are usually acidic and rich in plant matter with a conspicuous mat or living green moss. |
| Border dike | Earth ridge or small levee built to guide or to hold irrigation or recharge water in a field. |
| Border ditch | Small excavation used as a border of an irrigated strip or plot with water being spread from one or both sides. |
| Border irrigation | Irrigation by flooding strips of land, rectangular in shape and cross leveled, bordered by dikes. Water is applied al a rate sufficient to move it down the strip in a uniform sheet. Border strips having no downfield slope are referred to as level border systems. Border systems constructed on terraced lands are commonly referred to as benched borders. |
| Break grade | To change the slope of a pipe drain, ditch. or field. |
| Broad-crested weir | Weir water measurement having a rounded or wide crest in the direction of the stream. |
| Brush dam | Dam constructed of brush, in gullies or small channels, to retard flow of water and sediment. |
| Brush drain | Covered field drain consisting of a trench, the lower part of each section being filled with brush to form the drainage channel. |
| Bubbler irrigation | The application of water to flood the soil surface using a small stream or fountain. The discharge rates for point-source bubbler emitters are greater than for drip or subsurface emitters but generally less than 225 L/h (1 gpm). A small basin is usually required to contain or control the water. |
| Buffer strip | A strip of grass or other close-growing perennial vegetation, usually grass, that separates a watercourse from an intensive land-use area to prevent sediment entry into drainage channels. (Preferred term is Filter strip.) |
| Buffer strip cropping | An erosion control practice in which different crops are grown in consecutive strips across the slope and grass or close-growing vegetation is grown between the cultivated strips. |
| Bulk density | (Soil) The mass of dry soil per unit bulk volume. The bulk volume is determined before drying to constant weight at 105 °C (220 °F). |
| Bullet | (Drainage) Round-nosed cylindrical point of a mole drain plow which forms a cavity as the plow is drawn through the soil. (Also referred to as a Torpedo.) |
| Bypass ditch | A waterway for carrying water from a drainage area directly to a gravity outlet, bypassing any pumping plants. |
| C (index) | |
| Canopy | Vegetative cover over the land surface of a catchment area. |
| Canopy inlet | Entrance to a closed conduit that has been covered or shielded to induce priming al minimum submergence. |
| Canopy Interception | (Preferred term is Interception) |
| Capillary fringe | A zone in the soil just above the water table that remains saturated or almost saturated. The extent depends upon the size-distribution of pores. |
| Capillary pressure | (Preferred term is Soil-water pressure.) |
| Capillary pressure head | Height water will rise by surface tension above a free water surface in the soil, expressed as length unit of water. Sometimes called capillary rise. |
| Capillary soil moisture | (Preferred term is Soil-water potential.) |
| Capillary zone | (Preferred term is Capillary fringe.) |
| Capped riser or pot | An irrigation pipeline riser extending above ground, with a watertight cap over its top and outlet gates on its sides slightly above the ground surface (capped riser). To accommodate more outlet gates, a cap with a diameter larger than that of the riser pipe is sometimes installed on the top of the riser (capped pot). |
| Catch basin | Basin designed to catch or trap water and sediment. (Preferred term is Sediment basin.) |
| Catchment area or basin | A surface from which runoff is collected; watershed: drainage basin. |
| Centrifugal pump | Pump consisting of rotating vanes (impeller) enclosed in a housing and used to impart energy to a fluid through centrifugal force. |
| Channel capacity | Flow rate in a ditch, canal, or natural channel when flowing full or at design flow. |
| Channel improvement | Increasing the cross section, straightening, or clearing vegetation from a channel to change its hydraulic characteristics, increase its flow capacity, and reduce flooding. |
| Channel stabilization | Erosion prevention and stabilization of a channel by use of vegetation, jetties, drops, revetments, or other measures. |
| Channel storage | 1) (Hydrology) Water temporarily stored in channels while enroute to an outlet. 2) (Drainage) The volume of water that can be stored above the start pumping level in ditches or floodways without flooding cropland. |
| Check | Structure to control water depth in a canal, ditch. or irrigated field. |
| Check dam | Small barrier constructed in a gully or other small watercourse to decrease flow velocity, minimize channel scour, and promote deposition of sediment. |
| Check drain | Conventional drain altered by use of checks so that it can be used as a sub irrigation system. |
| Check irrigation | Modification of a border strip with small earth ridges or checks constructed at intervals to retain water as the water flows down the strip. |
| Chemical water treatment | 1) (Irrigation) Addition of acids, fungicides, and bactericides to prevent emitter clogging, adjust pH, or otherwise make the water acceptable for use in microirrigation systems. 2) (Drinking water) Addition of chemicals to disinfect, adjust pHm or sequester contaminates in drinking water. |
| Chemigation | Application of chemicals to crops through an irrigation system by mixing them with the irrigation water. |
| Chisel plowing | Cropland preparation by special implement (chisel) that minimizes inversion of the soil and leaves the soil surface relatively undistributed. |
| Chlorinated hydrcarbon | Synthetic compound that contains chlorine, hydrogen, and carbon; a main ingredient in some pesticides. |
| Chlorine residual | The concentration of chlorine remaining in water, sewage, or industrial wastewater following chlorination. |
| Christiansen's uniformity coefficient | A measure of the uniformity of irrigation water application. The average depth of irrigation water infiltrated minus the average absolute deviation from this depth, all divided by the average depth infiltrated. |
| Chute spillway | Lined channel constructed with a steep slope to convey water to a lower level without erosion. |
| Cipolleti weir | A sharp-crested trapezoidal weir with 1 (horizontal) to 4 (vertical) side slopes |
| Civil-law drainage rule | Drainage law stating that the owners of higher land are entitled to the natural advantage that the elevation of their land gives them and owners of lower lying land must receive surface water flowing to them through natural channels. |
| Clay | A soil separate consisting of particles less than 2 um in equivalent diameter. |
| Clay tile | Short lengths of pipe used for subsurface drains which are made from shale or clay. |
| Claypan | A dense, compact layer in the subsoil having a much higher clay content than the overlying material, separated by a sharply defined boundary. Claypans are usually hard when dry, and plastic and sticky when wet. Also, they usually impede the movement of water and air, and the growth of plant roots. |
| Climate | Meteorological elements that characterize the average and extreme conditions of the atmosphere over a long period of time at any one place or region of the earth's surface. |
| Climate change | The slow variations of climatic characteristics over time at a given place. |
| Closed drain | Subsurface drain, tile or perforated pipe, which may also receive surface water through surface inlets. |
| Coliform bacteria | A group of bacteria used as an indicator of sanitary quality in water. Exposure to these organisms in drinking water causes diseases such as cholera. |
| Colloids | Negatively charged soil particles smaller than 1 uum in diameter. |
| Colter slit | Knife-like cut or slit through the soil surface made when a mole plow is drawn through the soil or by a rolling disk. |
| Combined sewer | A sewer that receives storm water runoff as well as transports sewage. |
| Composting | Controlled microbial degradation of organic waste yielding an environmentally safe and nuisance-free soil conditioner and fertilizer. |
| Condensation | The process by which a vapor becomes a liquid or solid; the opposite of evaporation. In meteorological usage, this term is applied only to the transformation from vapor to liquid. |
| Cone index | The force per unit basal area required to push a cone penetrometer through a specified increment of soil. |
| Cone of depression or influence | The water table or piezometric, surface roughly conical in shape, produced by the extraction of water from a well. |
| Cone penetrometer | An instrument in the form of a cylindrical rod with a cone-shaped tip designed for penetrating soil and for measuring the end bearing component of penetration resistance. |
| Confined aquifer | An aquifer whose upper, and perhaps lower, boundary is defined by a layer of natural material that does not transmit water readily. |
| Conservancy district | A legal enterprise, organized for the purpose of accomplishing soil and water management and flood control. Sometimes called a natural resource district. |
| Conservation | The continuing protection and management of natural resources in accordance with principles that assure their optimum long-term economic and social benefits. |
| Conservation tillage | a tillage practice that leaves plant residues on the soil surface for erosion control and moisture conservation. |
| Consumptive use | The total amount of water taken up be vegetation for transpiration or building of plant tissue, plus the unavoidable evaporation of soil moisture, snow, and intercepted precipitation associated with vegetable growth. |
| Contaminant | Any physical, chemical, biological, or radiological substance or matter that has an adverse affect on air, water, or soil. |
| Continuous-flow irrigation | System of irrigation water delivery where each irrigator receives the allotted quantity of water continuously. |
| Contour farming | System of farming used for erosion control and moisture conservation whereby field operations are performed approximately on the contour. |
| Contour furrows | Furrows plowed approximately on the contour to reduce soil loss and increase infiltration. |
| Contour strip cropping | A modification of contour farming in which altering crops are planted in strips, usually includes strips of close growing, erosion resistant forage (grass, grain, or hay). |
| Contracted weir | Weir having sufficiently sharp upstream edges and slow approach velocity to cause the nappe to contract. Sometimes called a sharp-crested weir. |
| Control station | (Microirrigation) Facilities upstream of microirrigation distribution and application piping for purposes that may include water measurement, filtration, treatment, flow and pressure control, timing of application, and/or backflow prevention. Sometimes called a control head. |
| Control structure | Water regulating structure, usually for open conduits. |
| Controlled grainage | Regulation of the water table by means of pumps, control dams, or check drains, or a combination of these, for maintaining the water table at a depth favorable to crop growth. |
| Convective storm | Rainfall caused by condensation of warm moist air that moves upward, being cooled both by the surround air and by expansion. |
| Conventional tillage | The traditional tillage practice that involves inverting the tillage layer, burying most of the plant residues, and leaving the soil bare. |
| Conveyance loss | Loss of water from a channel or pipe during transport, including losses due to seepage, leakage, evaporation, and transpiration by plants growing in or near the channel. |
| Cooling tower | A structure that helps remove heat from water used as a coolant; e.g., in electric power generating plants. |
| Core wall | Masonry, sheet pilling, concrete, or compacted earth placed near the center of a dam or embankment in the form of a wall to reduce percolation. |
| Correction strip | The irregular width strips of land lying between uniform width strips in strip cropping systems. |
| Corrugated plastic tubing | Extruded plastic tubing with a corrugated wall and when perforated, used for subsurface drains. |
| Coupler | (Sprinkler) Device, either self-sealed or mechanically sealed, that connects, the ends of 2 lengths of pipe or pipe to hose. |
| Cover crop | Close growing crop, that provides soil protection, seeding protection, and soil improvement between periods of normal crop production, or between trees in orchards and vines in vineyards. When plowed under and incorporated into the soil, cover crops may be referred to as green manure crops. |
| Crack width | Space between the ends of adjacent clay or concrete drain tile. |
| Creep distance | The longitudinal length along the outer surface of a pipe or conduit plus the length along antisspe collars within an earth embankment. |
| Crest | 1) Top of a dam, dike, spillway, or weir. 2) Summit of a wave or peak of a flood. |
| Critical depth | Depth of flow in a channel at which specific energy is a minimum for a given discharge. |
| Critical suction head | (Preferred term in Set positive suction head.) |
| Critical velocity | Flow velocity at which a given discharge changes from tranquil to rapid or rapid to tranquil. That velocity in an open channel for which the specific energy is a minimum for a given discharge. |
| Crop area | (Irrigation) The field surface area allocated to each plant. The crop area is the plant spacing multiplied by the row spacing. |
| Crop irrigation requirement | Quantity of water, exclusive of effective precipitation, that is needed for crop production. |
| Crop residue | Portion of a plant, or crop, left in the field after harvest. |
| Crop rotation | A system of farming in which a succession of different crops are planted on the same land area, as opposed to growing the same crop time after time (monoculture). |
| Cross | (Sprinkler) Pipe fitting with 4 outlets or connections each 90 deg apart. |
| Crowning | The process of farming the surface of land into a series of broad, low ridges, separated by parallel field drains. |
| Cubic metre per second (m3/s) | A unit expressing rate of discharge, typically used in measuring stream flow. One cubic meter per second is equal to the discharge in a stream of a cross section one meter wide and one meter deep, flowing with an average velocity of one meter per second. |
| Curve number | An index of the runoff potential which is related to the soil and vegetation conditions of the site. Used in ScS Runoff Equation. |
| Cut | Portion of land surface or area from which earth or rock has been removed or will be removed by excavation the depth below original ground surface to excavated surface. |
| Cut-and-fill | Process of earthmoving by excavating part of an area and using the excavated material for adjacent embankments or fill areas. |
| Cutback irrigation | The reduction of the furrow or border inflow stream after water has advanced partially or completely through the field in order to reduce runoff. |
| Cutoff | 1) Wall, collar, or other structure such as a trench filled with relatively impervious material intended to reduce percolation of water along other wise smooth surfaces, or through porous strata. 2) A channel formed by a stream breaking through the neck of an oxbow. |
| Cutoff drain | (Preferred term is interceptor drain). |
| Cutslope | The Uphill side slope of a broad base-terrace channel. |
| D (index) | |
| Dam | A structure of earth, rock, concrete, or other materials designed to retain water, creating a pond, lake, or reservoir. |
| Darcy's Law | A concept formulated by Henry Darcy in 1856 to describe the rate of flow of water through a porous media is proportional to, and in the direction of, the hydraulic gradient and inversely proportional to the thickness of the bed. |
| Daylight | (Colloquid) The extension of a grade line to or above the ground surface. |
| Dead furrow | Empty furrow left when plowing. May be a single furrow when plowing in one direction or a double furrow when plowing in opposite direction. |
| Debris basin | Surface reservoir designed to trap sediment and debris. (Preferred term ins Sediment basin) |
| Debris cone | fan-shaped deposit of debris including soil, sand, gravel, and boulders built up at the point where a mountain stream meets a valley, or where the velocity of a stream is reduced sufficiently to cause such deposits. |
| Deep percolation | Water that moves downward through the soil profile below the root zone and cannot be used by plants. |
| Deep percolation percentage | The ratio of the average dept of irrigation water infiltrated and drained out of the root zone to the average depth of irrigation water applied, expressed as a percentage |
| Delivery box | (Irrigation) Structure diverting water or, a canal to a farm unit often including measuring devices. Also called turnout. |
| Delivery loss | (Preferred term is Conveyance loss) |
| Delta | A fan shaped alluvial deposit at a river mouth formed by the deposition of successive layers of sediment. |
| Demand | The numerical expression of the desire for goods and services associated with an economic standard for acquiring them. |
| Demand irrigation system | Irrigation water delivery procedure where each irrigator may request irrigation water in the amount needed and at the time desired. |
| Depletion | Loss of water from surface water reservoirs or groundwater aquifers at a rate greater than that of recharge. |
| Depletion curve | Recession curve for soil water, stream flow, ground water, etc. usually shown as a decay function with time. |
| Deposition | Transported material deposited because of decreased transport capacity of water or wind. |
| Depression storage | Water stored in surface depressions and therefore not contributing to surface runoff. |
| Di size | The particle size diameter such that i% (by weight) of a granular material is of smaller diameter. |
| Diameter of coverge | Average diameter of the area wetted by an irrigation sprinkler operating in still air. |
| Diffuse springs | Natural springs in which flow is not concentrated at one outlet. |
| Digestion | The microbial decomposition of organic matter under saturated conditions resulting in sludge that is a human-like mass and somewhat stable. |
| Dimension ratio | The ratio of the average pipe diameter to the minimum wall thickness. The pipe diameter may be either outside or inside diameter. |
| Dioxin | Any of a family of compounds known chemically as dibenzo-p- dioxins. Concern about them arises from their potential toxicity and contamination in commercial products. |
| Dipper dredge | Excavating machine using a boom-type shovel. |
| Discharge | In the simplest form, discharge means outflow of water. The use of this term is not restricted as to course or location and it can be used to describe the flow of water from a pipe or from a drainage basin. Other words related to it are runoff, stream flow, and yield. |
| Discharge bay | Enlarged channel section for collecting the discharge of a pumping plant. |
| Discharge coefficient | Ratio of observed flow to theoretical flow. |
| Discharge curve | Rating curve showing the relation between stage and flow rate of a stream, channel or conduit. |
| Displacement meter | Meter that measures the quantity of flow by recording the number of times a container of known volume is filled and emptied. Used primarily for low flow measurement. |
| Dissolved oxygen (DO) | The amount of oxygen freely available in water and necessary for aquatic life and the oxidation of organic materials. |
| Dissolved solids (DS) | Very small pieces of organic and inorganic material contained in water. Excessive amounts make water unfit to drink or limits its use in industrial processes. |
| Distributary | 1) A small conduit or channel taking water from a canal for delivery to farms. 2) Any system of secondary conduits. 3) River channel flowing away from the main stream and not rejoining it, as contrasted to a tributary. |
| Distribution system | System of ditches, or conduits and their controls, which conveys water from the supply canal to the farm points of delivery. |
| Distribution uniformity | Measure of the uniformity of irrigation water distribution over a field. |
| Distribution uniformity of low quarter | The ratio of the average of the lowest one-fourth measurements of irrigation water infiltrated to the average depth of irrigation water infiltrated, expressed as a percent. |
| Ditch storage | (See Channel storage.) |
| Diversion | A channel or dam constructed across a slope to intercept surface runoff and divert it to a safe or convenient discharge point. Usually placed above the area to be protected. |
| Diversion box | Structure built into a canal or ditch for dividing the water into predetermined portions and diverting it to other canals or ditches. |
| Diversion dam | Barrier built in a stream for the purpose of diverting part or all the water from the stream into a canal. |
| Doctrine of appropriation | Legal doctrine of water rights that asserts that all claims to water are based on beneficial use. |
| Domestic use | The quantity of water used for household purposes such as washing, food preparation, and bathing. |
| Double ditch or drain | (See W-ditch.) |
| Double-main system | Gridiron layout of subsurface drains with 2 closely-spaced parallel main conduits. |
| Drain | Any closed conduit (perforated tubing or tile) or open channel, used for removal of surplus ground or surface water. |
| Drain inlet structure | (See Surface inlet.). |
| Drain plow | A machine with a vertical blade. chisel point, and shield or boot used to install corrugated plastic tubing or drain tile. |
| Drain tile | Short length of pipe made of burned clay, concrete, or similar material, usually laid with open joints, to collect and remove subsurface water. |
| Drainage | Process of removing surface or subsurface water from a soil or area. |
| Drainage basin | The area from which runoff is collected and delivered to an outlet. |
| Drainage coefficient | Rate at which water is to be removed from a drainage area, expressed as depth per day or flow rater per unit of area. Sometimes called “drainage modulus”. |
| Drainage curves | Flow rate versus drainage area curves giving prescribed rates of runoff for different levels of crop protection. |
| Drainage pattern | 1) Arrangement of a system of surface or subsurface drains. 2) Arrangement of tributaries within a watershed. |
| Drainage pumping plant | Pumps, power units, and appurtenances for lifting drainage water form a collecting basin to an outlet. |
| Drainage system | Collection of surface and/or subsurface drains, together with structures and pumps, used to remove surface or ground water. |
| Drainage well | 1) A well pumped in order to lower water tables. 2) Vertical shaft to a permeable substratum into which surface and subsurface drainage water is channeled (now illegal.). |
| Drawdown | 1) Lowering of the water surface, water table, or piezometric surface resulting from the withdrawal of water from a well or drain. 2) The elevation of the static water level in a well minus the elevation of the pumping water level (at the well) at a given discharge rate. (See Cone of depression.) |
| Dredging | The removal of mud from the bottom of water bodies using a scooping machine. This disturbs the ecosystem and causes silting that can kill aquatic life. |
| Drip irrigation | A method of microirrigation wherein water is applied to the soil surface as drops or small streams through emitters. Discharge rates are generally less than 8 L/h (2 gal/h) for a single-outlet emitters and 12 L/h (3 gal/h) per meter for line-source emitters. |
| Drop spillway | Overfall hydraulic structure in which the water drops over a vertical wall onto an apron. |
| Drop structure | Hydraulic structure for safety transferring water in a channel to lower level channel without causing erosion. |
| Drop-inlet spillway | Overfall hydraulic structure in which water is discharged through a vertical riser conduit. |
| Drought | A continuous and lengthy period during which no significant precipitation is recorded. |
| Dry deposition | Emissions of sulphur and nitrogen oxides which, in the absence of water in the atmosphere (i.e., rain), settle to the ground as particulate matter. |
| Duty of water | (See Crop irrigation equipment.) |
| Dyke | An artificial embankment constructed to prevent flooding. |
| Dynamic head | Specific energy in a flow system. |
| E (index) | |
| Ecosystem | A system formed by the interaction of a group of organisms and their environment. |
| Effective precipitation | That portion of total precipitation which becomes available for plant growth. |
| Effluent | The sewage or industrial liquid waste which is released into natural water by sewage treatment plants, industry, or septic tanks. |
| Effluent irrigation | Land application of wastewater for irrigation and beneficial use of nutrients. |
| Effluent stream | Stream or part of a stream that receives water from ground water or seepage. (Preferred term is Gaining stream.) |
| Electrical conductivity | A measure of the ability of water to conduct electricity, which is used to estimate the amount of soluble salts in irrigation or drainage water, or solution extract of soil. |
| Electrical resistance block | Small block consisting of electrodes set in an absorbent material, such as gypsum, used to estimate soil water content. |
| Elevation head | Energy possessed by a fluid due to its position above some datum. |
| Emergency spillway | Auxiliary channel which transmits floodwater exceeding the capacity of the principal spillway. |
| Emission point | The location where water is discharged from an emitter. |
| Emission uniformity | An index of the uniformity of emitter discharge rates through a microirrigation system. Takes account of both variations in emitters and variations in the pressure under which they operate. |
| Emitter | A small microirrigation dispensing device designed to dissipate pressure and discharge a small uniform flow or trickle of water at a constant discharge, which does not vary significantly because of minor differences in pressure head. Also called a “dripper” or “trickler”. |
| Emitter - Compensating emitter | Designed to discharge water at a constant rate over a wide range of lateral line pressures. |
| Emitter - Continuous flushing emitter | Designed to continuously permit passage of large solid particles while operating at trickle or drip flow thus reducing filter fineness requirements. |
| Emitter - Flushing emitter | Designed to have a flushing flow of water to clear the discharge opening every time the system is turned on. |
| Emitter - Line-source emitter | Water is discharged from closely spaced perforations, emitters, or a porous wall along the tubing. |
| Emitter - Long path emitter | Employs a long capillary sized tube or channel to dissipate pressure. |
| Emitter - Multi-outlet emitter | Supplies water to 2 or more points through small diameter auxiliary tubing. |
| Emitter - Orifice emitter | Employs a series of orifices to dissipate pressure. |
| Emitter - Vortex emitter | Employs a vortex effect to dissipate pressure. |
| Entrance head | Head required to establish flow into a conduit or structure. |
| Entrance loss | Energy lost in eddies and friction at the inlet to a conduit or structure. |
| Envelope filter | Granular material or geotextile fabric which surrounds a subsurface pipe drain to prevent soil inflow and enhance eater entry. |
| Environment | All of the external factors, conditions, and influences which affect an organism or a community. |
| Environmental impact assessment | The critical appraisal of the likely effects of a proposed project, activity, or policy on the environment, both positive and negative. |
| Environmental monitoring | The process of checking, observing, or keeping track of something for a specified period of time or at specified intervals. |
| Ephemeral gully | Small channels eroded by runoff which can be easily filled and removed by normal tillage, only to reform again in the same location. |
| Equalizing ditch | Secondary ditch, usually parallel to a field ditch, used to furnish irrigation water to 2 or more furrows. |
| Erosion | The wearing away of the land surface by running water, wind, ice, or other geological agents, including such processes as gravitational creep. The following terms are used to describe the different types of water erosion: |
| Erosion - Accelerated erosion | Erosion much more rapid that normal, natural, or geological erosion, primarily as a result of the influence of the activities of man or, in some cases, of animals. |
| Erosion - Geological erosion | The normal or natural erosion caused by geological processes acting over long geological periods. (Synonymous with Natural erosion.) |
| Erosion - Gully erosion | The erosion process whereby water accumulates in narrow channels and, over short periods, removes the soil from this narrow area to considerable depths, ranging from 0.5 m (1.6 ft.) to as much as 30m (97 ft.). |
| Erosion - Interrill erosion | The removal of a fairly uniform layer of soil on a multitude of relatively small areas by splash due to raindrop impact and by shallow surface flow. |
| Erosion - Natural erosion | Wearing away of the earth’s surface by water, ice, or other natural agents under natural environmental conditions of climate, vegetation, etc., undisturbed by man. (See Geological erosion.) |
| Erosion - Normal erosion | The gradual erosion of land used by man which does not greatly exceed natural erosion. (See Natural erosion.) |
| Erosion - Rill erosion | An erosion process in which numerous small channels of only several centimeters in depth are formed; occurs mainly on recently cultivated soils. (See Rill.) |
| Erosion - Sheet erosion | The removal of soil from the land surface by rainfall and surface runoff. Often interpreted to include rill and interrill erosion. |
| Erosion - Splash erosion | The detachment and airborne movement of small soil particles caused by the impact of raindrops on soils. |
| Erosion class | Numerical ranking of erosion or erosion potential obtained in a soil conservation survey. |
| Erosion pavement | A layer of coarse fragments, such as sand or gravel, remaining on the surface after removal of fine particles of erosion. |
| Erosion potential | A numerical value expressing the inherent erodibility of a soil or maximum potential soil. |
| Erosive velocity | Velocity of the erosive agent necessary to cause erosion. |
| Erosivity | The potential ability of water, wind , gravity, etc., to cause erosion. |
| Erosivity index | A relative value of erosion potential based primarily on rainfall. (See Erosion potential.) |
| Estuarine inflow | The freshwater input necessary to provide nutrient input, sediment movement, circulation and maintenance of brackish conditions for estuarine organisms. |
| Estuary | Regions of interaction between rivers and nearshore ocean waters, where tidal action and river flow create a mixing of fresh and salt water. These areas may include bays, mouths of rivers, salt marshes, and lagoons. These brackish water ecosystems shelter and feed marine life, birds, and wildlife. |
| Eutrophic lake | Shallow, murky bodies of water that have excessive concentrations of plant nutrients causing excessive algal production. |
| Eutrophication | The natural process by which lakes and ponds become enriched with dissolved nutrients, resulting in increased growth of algae and other microscopic plants. |
| Evaporation | The process by which a liquid changes to a vapor. |
| Evapotranspiration | The combination of water transpired from vegetation and evaporated from the soil and plant surfaces. |
| Exchange capacity | The total ionic charge of the absorption complex active in the adsorption of ions. Field application duration: The elapsed time form the beginning of water application to the first irrigation set to the time at which water application is terminated on the last irrigation set of a field. |
| Exchangeable cation | A positively charged ion held on or near the surface of a solid particle by a negative surface charge of a colloid and which may be replaced by other positively charged ions in the soil solution. |
| Exchangeable sodium percentage | The fraction of the cation exchange capacity of a soil occupied by sodium ions. |
| F (index) | |
| Fen | A type of wetland that accumulates peat deposits. Fens are less acidic than bogs, deriving most of their water from groundwater rich in calcium and magnesium. |
| Field capacity | Amount of water remaining in a soil when the downward water flow due to gravity becomes negligible. |
| Field ditch | A ditch constructed within a field either for irrigation or drainage. |
| Field drain | A shallow-graded channel, usually having relatively flat side slopes, that collects surface water within a field. |
| Field lateral | The principal ditch for draining areas on a farm. Field laterals receive water from row drains, field drains, and field surfaces and carry it to the farm mains. |
| Filter | 1) (Drainage) Envelope of graded porous material placed around a closed drain to prevent soil from entering the drain. 2) (Wells) Sands, gravels, or fibrous materials placed around a well screen or perforated casing to increase permeability near the well and prevent unwanted aquifer particles from entering the well. 3) (Irrigation) Device used in micro and sprinkler irrigation systems to remove debris from the water that might clog or otherwise foul the emitters or sprinklers. |
| Filter strip | Permanent vegetated strip between fields and receiving waters or runoff conveyance structures to retard surface runoff and remove sediment, nutrients, on other contaminates from surface runoff. |
| Final infiltration rate | (See Basic intake rate.) |
| Flashboard | Wood plank, generally held horizontally in vertical sots on the crest of a dam or check structure to control the upstream water level. Sometimes called “crestboard”. |
| Flexible membrane liner | Synthetic barrier to water movement commonly fabricated of polyvinyl chloride or high density polyethylene sheets used to line water storage facilities to prevent seepage. |
| Float valve | A valve actuated by a float, which automatically controls the flow of water. |
| Flood | The temporary inundation of normally dry land areas resulting from the overflowing of the natural or artificial confines of a river or other body of water. |
| Flood control | Methods or facilities for controlling flood flows. |
| Flood damage | The economic loss caused by floods, including damage by inundation, erosion, and/or sediment deposition. Damages also include emergency costs and business or financial losses. Evaluation may be based on the cost of replacing, repairing, or rehabilitating; or the comparative change in market or sales value; or on the change in the income or production caused by flooding. |
| Flood forecasting | Prediction of stage, discharge, time of occurrence and duration of a flood, especially of peak discharge at a specified point on a stream, resulting from precipitation and/or snowmelt. |
| Flood fringe | The land on which water is stored as dead water during flooding, and which does not contribute to the downstream passage of flow. |
| Flood gate | Mechanical gate to prevent backflow into a closed conduit during high water stages. Sometimes called “drainage gate” |
| Flood irrigation | Method of irrigation where water is applied to the soil surface without flow controls, such as furrows, borders or corrugations. |
| Flood peak | The highest magnitude of the stage of discharge attained by a flood. Also called peak stage or peak discharge. |
| Flood routing | Process of determining stage height, storage volume, and outflow from a reservoir or reach of a stream for a given hydrograph of inflow. |
| Flood spillway | An auxiliary channel to carry a flood flow that exceeds a given design rate to the channel downstream. |
| Flood water retarding structure | Barrier across a watercourse usually designed with a restricted outlet for the temporary storage of runoff and release over time. |
| Floodplain | Any normally dry land area that is susceptible to being inundated by water from any natural source. This area is usually lowland adjacent to a stream or lake. |
| Floodplain storage | Volume of water that spreads out and is temporally stored in a floodplain. |
| Floodproofing | Any combination of structural and nonstructural additions, changes, or adjustments to structures which reduce or eliminate flood damage. |
| Floodway | The channel of a river or stream and those parts of the adjacent floodplain adjoining the channel which are required to carry and discharge the base flood. |
| Flow | The rate of water discharged from a source; expressed in volume with respect to time, e.g., m3/s. |
| Flow augmentation | The addition of water to a stream especially to meet instream flow needs. |
| Flow line | Lowest level of flow in a conduit or channel. |
| Flume | 1) Open conduit for conveying water across obstruction. 2) An entire canal elevated above natural ground. An aqueduct. 3) A specially calibrated structure for measuring open channel flows. |
| Food chain | A sequence of organisms, each of which uses the next, lower member of the sequence as a food source. |
| Food web | The complex intermeshing of individual food chains in an ecosystem. |
| Forced outlet | Basin or box outlet for a pipe drain in which the discharge will fill the basin and flow away over the ground surface. Used where a free fall outlet is not available. |
| Forebay | Reservoir or pond at the intake of a penstock, pipeline, or pump station. |
| Forest floor | Litter humus, and organic matter which lies on the mineral soil surface under forest vegetation. |
| Free discharge | Discharge of water from a conduit into the atmosphere without backpressure. |
| Free Flow | Flow through or over a structure without backpressure. |
| Freeboard | Vertical distance between the maximum water surface elevation anticipated in design and the top of retaining banks, pipeline vent, or other structures, provided to prevent overtopping because of unforeseen conditions. |
| French drain | An excavated trench refilled with previous materials, through whose voids water flows toward an outlet. |
| Fresh water | Water that generally contains less than 1000 milligrams per liter of dissolved solids such as metals, nutrients, etc. |
| Friction head | Energy required to overcome friction caused by fluid movement relative to the boundaries of a conduit or containing medium. |
| Friction slope | Friction head loss per unit length of conduit. |
| Frost depth | The depth to which a soil will freeze. |
| Full irrigation | Management of water application to fully replace the soil water deficiency over an entire field. |
| Furrow | 1) A trench in the soil made by a tillage tool. 2) Small channel for conveying irrigation water. |
| Furrow dike | Small dike formed in a furrow to prevent runoff. |
| Furrow irrigation | Method of surface irrigation where the water is supplied too small ditches or furrows for guiding across the field. |
| G (index) | |
| Gabion | Rectangular or cylindrical wire mesh cage filled with rock for protecting aprons, stream banks, shorelines, etc., against erosion. |
| Gaining stream | Stream or part of a stream that has an increase in flow because of inflow from ground water. |
| Gallery | Underground chamber constructed for collection of ground water. |
| Gate | A device used to control the flow of water to, from, or in a pipeline, or open channel. It may be opened and closed by screw action, slide action, or hydraulic or pneumatic actuators. |
| Gated pip | Portable pipe with small gates installed along one side for distributing irrigation water to corrugations or furrows. |
| Gauge height | 1) (Surveying) The vertical distance from the sight bar, batter board or receiver to the bottom of the finished cut. 2) (Hydraulics) Elevation of a water surface measured by a gauge. |
| Gauging Station | Section in a stream channel equipped with a gauge or facilities for obtaining stream flow data. |
| Geographic information systems, GIS | Computer database management system for spatially distributed attributes. |
| Geotextile | Fabric or synthetic material placed between the soil and pipe, gabion, or retaining wall to 1) enhance water movement and retard soil movement, and 2) as a blanket to add reinforcement and separation. |
| Glacier | A huge mass of ice, formed on land by the compaction and re- crystallization of snow, that moves very slowly downslope or outward due to its own weight. |
| Grade | (noun) Degree of slope of a road, channel, or ground surface. (verb) To finish the surface of a canal bed, roadbed, top of embankment, or bottom of excavation. |
| Grade breaker | A special mechanical device attached to an earthmoving machine to change the normal grade line. |
| Grade control | The process of maintaining constant and correct slope of a trench, ditch, terrace, canal, etc., using optical or laser surveying equipment. |
| Grade line | A line established as a construction reference for ditches terraces, etc. |
| Grade stabilizing structure | Structure used to control the bottom grade of a channel. |
| Graded terrace | A terrace with sloping channel, constructed to reduce field slope lengths and remove runoff, with erosion control as the primary objective. |
| Gradually varied flow | Steady non-uniform open channel flow in which the changes in depth and velocity from section are gradual enough such that accelerative forces are negligible. |
| Grassed waterway | Natural or constructed channel covered with an erosion-resistant grass that transports surface runoff to a suitable discharge point at a non-erosive rate. |
| Grated inlet | A specific type of surface inlet to a pipe drain projected with a grate. |
| Gravel fillter | 1) Graded sand and gravel aggregates placed around a subsurface drain (called a gravel envelope). 2) Well screen to prevent the infiltration of fine materials into the drain or well. |
| Gravitational water | Soil water, which moves into, through, or out of the soil under the influence of gravity. Used in irrigation, drainage, inlets, and outlets. |
| Greenbelt | A strip of land kept in its natural state or in agricultural use to break up the continuous pattern of urban development. |
| Greenhouse effect | The warming of the earth's atmosphere caused by a build-up of carbon dioxide or other trace gases; it is believed by many scientists that this build-up allows light from the sun's rays to heat the earth but prevents a counterbalancing loss of heat. |
| Grey water | Domestic wastewater other than those containing human excreta such as sinks drainage, washing machine discharge, or bath water. |
| Gridiron system | Arrangement of a drainage system where parallel laterals enter a main on one side only. |
| Gross diversion requirement | The total quantity of water diverted from a stream, lake, or reservoir, or removed from the ground in order to irrigate a crop. |
| Ground water | Water occurring in the zone as saturation in an aquifer or soil. |
| Ground water flow | Flow of water in an aquifer or soil. That portion of the discharge of a stream that is derived from ground water. |
| Ground water mining | Pumping of ground water, for irrigation or other uses, at rates significantly faster than the rate which the ground water us being recharged. Ultimately the aquifer is depleted. |
| Groundwater recharge | The inflow to an aquifer. |
| Growing season | The period, often the frost-free period, during with the climate is such that crops can be produced. |
| Gully | Eroded channel where runoff concentrates, usually so large that it cannot be obliterated by normal tillage operations. |
| Gully head advance | Upstream migration of the upper end of a gully. |
| Gypsum block | An electrical resistance block in which the absorbent material is gypsum. |
| H (index) | |
| Habitat | The native environment where a plant or animal naturally grows or lives. |
| Hardpan | (soil) A hardened soil layer, in the lower A or B-horizon, caused by cementation of soil particles. |
| Hazardous waste | Waste that poses a risk to human health or the environment and requires special disposal techniques to make it harmless or less dangerous. |
| Head | The energy in the liquid system expressed as the equivalent height of water column above a given datum. |
| Head ditch | Ditch across the upper end of a field used for distributing water in surface irrigation. |
| Head gate | Water control structure from a structure. 2) Source of a stream. |
| Head works | Diversion structures at the upper end of a conduit or canal. |
| Herringbone system | Arrangement of a pipe drainage system where laterals enter a main from both sides at angles less than 90 deg. |
| Hood stress | The tensile stress in the wall of a pipe in the circumferential orientation caused by internal hydrostatic pressure. (See Hydrostatic design stress). |
| Hooded inlet | (see Canopy inlet) |
| Humid climates | Climate characterized by high rainfall and low evaporation potential. A region is usually considered as humid when precipitation averages more than 500 mm (20 in.) per yr. |
| Hydrant | An outlet, usually portable, used for connecting surface irrigation pipe or an alfalfa valve outlet. |
| Hydraulic radius | Cross sectional area of a fluid stream or conduit divided by its wetted perimeter (length of its conduit surface in contact with fluid). |
| Hydraulic conductivity | The ability of a porous medium to transmit a specific fluid under a unit hydraulic ingredient; a function of both the characteristics of the medium and the properties of the fluid being transmitted. Usually a laboratory measurement corrected to a standard temperature and expressed in united of length/time. Although the term hydraulic conductivity is sometimes used interchangeably with the term permeability, the user should be aware of differences. |
| Hydraulic efficiency | 1) Efficiency with which a pump imparts energy to water or a turbine extracts energy from water. 2) A Measure of the loss of energy when water flows through a hydraulic structure. |
| Hydraulic gradient | Change in the hydraulic head per unit distance. |
| Hydraulic jump | Abrupt turbulent rise in water level from a flow stage less than critical depth to a flow stage greater than critical depth within which the velocity passes from supercritical to subcritical. |
| Hydraulic ram | Device that uses the energy of flowing water to lift a portion of the flow. |
| Hydraulic resistance | Friction along the wetted boundary of a channel or conduit with respect to time. |
| Hydroelectricity | Electric energy produced by water- powered turbine generators. |
| Hydrograph | Graphical or tabular representation of the flow rate of a stream with respect to time. |
| Hydrologic condition | An indication of the effects of ground cover and treatment on infiltration and runoff generally estimated from the density of plants and residue cover on sample areas. |
| Hydrologic cycle | Term used to describe the movement of water in and on the earth and atmosphere. Numerous processes such as precipitation, evaporation, condensation, and runoff comprise the hydrologic cycle. |
| Hydrology | The science of waters of the earth; water's properties, circulation, principles, and distribution. |
| Hydrostatic pressure | Force per unit of area exerted by a liquid at rest. |
| Hygroscopic moisture | Moisture absorbed by dry soil from a saturated atmosphere. |
| I (index) | |
| Impact threshold velocity | Minimum velocity required to initiate soil movement by impact of particles carried by wind or water. |
| Impeller meter | A rotating mechanical device for measuring flow rate in a pipe or open channel. |
| Impermeable layer | (soil) Layer of soil resistant to penetration by water, air, or roots. |
| Infilltrometer | Device for measuring the infiltration rate. |
| Infiltration | The downward entry of water through the soil surface into the soil. |
| Infiltration opportunity time | The time that water inundates the soil surface, with opportunity to infiltrate. |
| Infiltration rate | The quantity of water that enters the soil surface I a specified time interval. Often expressed in volume of water per unit of soil surface area per unit of time. |
| Inflow | The entry of extraneous rain water into a sewer system from sources other than infiltration, such as basement drains, manholes, storm drains, and street washing. |
| Influent stream | Stream or portion of stream that contributes water to the ground water supply. |
| Initial storage | That portion of precipitation required to satisfy interception, the wetting of the soil surface, and depression storage sometimes called “initial abstraction”. |
| Inlet | 1) An appurtenance to deliver water to a pipeline system. 2) Point of defined inflow into a conduit or channel. |
| Inorganic | Matter other than plant or animal, and not containing a combination of carbon/hydrogen/oxygen as in living things. |
| Instream flow requirements | The flow regime necessary to provide for the combined needs of fish, wildlife, recreation, navigation, hydropower production, and downstream conveyance in a stream. |
| Instream use | Uses of water within the stream channel; e.g., fish and other aquatic life, recreation, navigation, and hydroelectric power production. |
| Intake | 1) Head-works of a conduit. 2) The place of diversion. 3) Water infiltration into soil. |
| Integrated resource planning | The management of two or more resources in the same general area; commonly includes water, soil, timber, grazing land, fish, wildlife, and recreation. |
| Interbasin transfer | The diversion of water from one drainage basin to one or more other drainage basins. |
| Interception | That portion of precipitation caught by vegetation and prevented from reaching the soil surface. |
| Interceptor drain | A channel located across the flow of ground water and installed to collect subsurface flow before it resurfaces. Surface water is also collected and removed. |
| Intercropping | Planting 2 or more crops in the same land area at the same time often used to help control pest populations that often occur on monoculture crops. Sometimes called “polycropping” or “plant stratification”. |
| Interflow | Water that infiltrates into the soil and moves laterally through the upper soil horizon until it returns to the surface, often in a stream channel. |
| Interior dike | Secondary dike in a multiple dike flood protection system. |
| Interior drain | Drain installed within a dam or other earth structure as distinguished from peripheral or interceptor drains. |
| Interlock injection device | Safety equipment used to ensure that a chemical injection pump will stop if the irrigation pumping plant stops to prevent the entire chemical mixture from emptying from the supply tank into the irrigation pipeline. An injection device may include check valves to prevent water from flowing back through the injection pump and overflowing the chemical supply tank, whenever the injection pump is turned off and the irrigation pump is still operating. |
| Intermittent stream | Natural channel in which water does not flow continuously. |
| Internal drainage | Drainage of the soil profile; may be either natural or augmented by men. |
| Interstices | Spaces between soil particles or aggregates filled with water or air. |
| Intrinsic permeability | The property of a porous material that expresses the ease with which gases or liquids flow through it. |
| Inversion | An increase in air temperature with an increase in altitude above the earth, resulting in increased stability of the atmosphere. |
| Invert | Lowest element of the internal crosses section of a channel or pipe. |
| Inverted siphon | A closed conduit with end sections above the middle sections; used for crossing below a depression or under a highway. |
| Irrecoverable water loss | Water loss that becomes unavailable for reuse through evaporation, phreatophyte transpiration, or ground water recharges that is not economically recoverable. |
| Irrigable area | Area capable of being of being irrigated, principally as regards to availability of water, suitable soils, and topography of land. |
| Irrigating stream | 1) Flow for irrigation of a particular tract of land. 2) Flow of water distributed at a single irrigation. Sometimes called “irrigating head”. |
| Irrigation | The controlled application of water to cropland, hayland, and/or pasture to supplement that supplied through nature. |
| Irrigation check | Small dike or dam used in the furrow alongside an irrigation border to make the water spread evenly across the border. |
| Irrigation district | A cooperative, self-governing semipublic organization set up as a subdivision of a state or local government to provide irrigation water. |
| Irrigation efficiency | The ratio of the average depth of irrigation water that is beneficially used to the average depth of irrigation water applied, expressed as a percent. Beneficial uses include satisfying the soil water deficit and any leaching requirement to remove salts from the root zone. |
| Irrigation hose | A closed conduit for supplying water to moving irrigation systems, flexible when subjected to normal operating pressure and may be collapsible to a flat cross section when purged of water. |
| Irrigation interval | The average time interval between the commencement of successive irrigation for a given field. Sometimes called “irrigation frequency”. |
| Irrigation runoff percentage | The equivalent depth of irrigation water running off a field expressed as percentage of the depth of irrigation water applied. |
| Irrigation set | The area irrigated at one time within a field. |
| Isotropic | (soil) The condition of a soil or other porous media when physical properties, particularly hydraulic conductivity, are equal in all directions. |
| J (index) | |
| Jetty | A wall or dike built of piles, rock, or other material, extending into a stream or into the sea at the mouth of a river to induce scouring or deposition or for erosion protection. |
| Joint wrapping | Placement of porous material over or around the pipe joints of subsurface drains to help prevent inflow of seediness. |
| Jökulhlaup | Destructive flood that occurs as the result of the rapid ablation of ice by volcanic activity beneath the ice of a large glacier. |
| Junction | 1) Point of intersection of 2 drains. 2) Accessory used to create a connection between 2 pipelines. |
| Junction box | Box, manhole, or other structure which serves to join 2 or more pipes. |
| K (index) | |
| Katerak | 1) Secondary or side channel ditch or conduit Also called “branch drain” or “spur”. 2) Water delivery pipeline that supplies irrigation water from the main line to sprinklers or emitters. |
| Keel | Longitudinal strip attached at the center bottom of the shoe of a trenching machine to form the trench bottom. |
| Key terrace | Terrace that is selected as a reference in laying out other terraces. |
| Kilowatt (kW) | A unit of electrical power equal to 1000 watts or 1.341 horsepower. |
| Kilowatt hour (kWh) | One kilowatt of power applied for one hour. |
| Kinematic wave | A method of mathematical analysis of unsteady open channel flow in which the dynamic terms are omitted because they are small and assumed to be negligible. |
| L (index) | |
| Lag time | 1)(Hydrology) The interval between the time when one half of the equivalent uniform excess rain (runoff) has fallen and the time when the peak of the runoff hydrograph occurs. 2) (Irrigation) The interval after water is turned off at the upper end of a field, until it recedes (disappears) from that point. |
| Lagoon | (1) A shallow pond where sunlight, bacterial action, and oxygen work to purify wastewater. (2) A shallow body of water, often separated from the sea by coral reefs or sandbars. |
| Lake | Any inland body of standing water, usually fresh water, larger than a pool or pond; a body of water filling a depression in the earth's surface. |
| Lamina | Thin plate or scale, layer, or flake as in a soil that is made up of thin flakes. |
| Laminar flow | Flow in which there are no crosscurrents or eddies, and where the fluid elements move in approximately parallel directions. Flow through granular materials is usually laminar. Sometimes called “streamline” or “viscous flow”. |
| Land capability | Classification of soil units for the purpose of showing their relative suitability for specific uses, such as crop production with minimum erosion hazard. |
| Land grading | The operation of shaping the surface of land to predetermined grades so each row or surface slopes to a drain or is configured for efficient irrigation water applications. Also called “land forming” or “land shaping”. |
| Land leveler | A machine with a long wheelbase used for land smoothing or leveling operations. |
| Land leveling | Process of shaping the land surface to a level surface. A special case of lands grading. |
| Land-use planning | Development of plans for the use of land that will, over a long period, best serve the interest of the general public. |
| Laser receiver | An electronic device normally mounted on earth moving machines or trenchers which receiver signal from a laser transmitter and indicates to the operator or sends signal to control points on the machine to adjust the machine to follow the slope established by a laser transmitter. |
| Laser transmitter | A device, which generates the collimated laser, light beam. |
| Lath box | A wooden box that is placed in a ditch bank to transfer water from an irrigation ditch to the field to be irrigated. |
| Leaching | Removal of soluble material from soil or other permeable material by the passage of water through it. |
| Leaching fraction | The ratio of the depth of subsurface drainage water (deep percolation) to the depth of infiltrated irrigation water. |
| Leaching requirement | Quantity of irrigation water required for transporting salts through the soil profile to maintain a favorable salt balance in the root zone for plant development. |
| Length of run | Distance water must flow in furrows or borders over the surface of a field from he head to the end of the field. |
| Lenker rod | A level rod used for surveying that has a loop graduated tape or ribbon from which elevations can be read directly. |
| Lethal dose | The amount of a toxic substance required to cause death of an organism under study in a given period. |
| Limited irrigation | Management of irrigation applications to apply less than enough water to satisfy the soil water deficiency in the entire root zone. Some times called “deficit” or “stress irrigation’s”. |
| Limited tillage | Tillage practices that leave plant residence on the soil surface for erosion control and moisture conservation. |
| Line gate | A hub-end screw-type valve that is installed in a pipeline. |
| Line source | Continuous source of water emitted along a line. |
| Lining | Protective covering over the perimeter of a conduit, reservoir, or channel to prevent seepage losses, to withstand pressure, or to resist erosion. |
| Litre | The basic unit of measurement for volume in the metric system; equal to 61.025 cubic inches or 1.0567 liquid quarts. |
| Longitudinal smoothing | Land smoothing operation where all soil movement is done parallel to crop row direction for the purpose of obtaining a grade. |
| Long-term hydrostatic strength | The estimated tensile stress of a pipe wall in the circumferential orientation that, when applied continuously, will cause failure of the pipe at 100,000h. |
| Losing stream | A channel that loses water into the bed or banks. |
| Lysimeter | An isolated block of soil, usually undisturbed and in situ, for measuring the quantity, quality, or rate of water movement through or from the soil. |
| M (index) | |
| Management allowed depletion | The desired soil water deficit at the time of irrigation. |
| Manifold | Pipeline that supplies water to the laterals. |
| Manometer | Instrument which measures the pressure of liquids and gases by fluid displacement. |
| Manufacturer’s coefficient of variation | A measure of the variability of discharge of a random sample of a given make, model, and size of microirrigation emitter, as produced by the manufacturer and before any field operation or aging has taken place; equal to the ratio of the standard deviation of the discharge of the emitters to the mean discharge of the emitters. |
| Marsh | A type of wetland that does not accumulate appreciable peat deposits and is dominated by herbaceous vegetation. Marshes may be either fresh or salt water and tidal or non-tidal. |
| Mattress | 1) Mat or weighted blanket constructed of brush or poles, interwoven or other wise lashed together, placed in a channel to control erosion. 2) A wire basket, less than 30 cm (12 in) thick, filled with stones. |
| Mean depth | Cross sectional area of a stream divided by its surface width the average depth. |
| Mean velocity. | Velocity obtained by dividing the flow rate by the cross sectional area: the average velocity. |
| Megawatt | A unit of electricity equivalent to 1000 kilowatts. |
| Meter gate | A calibrated irrigation valve used for flow measurement. |
| Microirrigation | The frequent application of small quantities of water as drops, tiny streams, or miniature spray through emitters or applicators placed along a water delivery line. Microirrigation encompasses a number of methods or concepts such as bubbler, drip, trickle, mist, or spray. |
| Miner’s inch | Discharge from an orifice 1-in. under a definite head, fixed by statute or practice but differing from state to state. In Colorado miner’s inch equals 0.00074 m3/s. In Arizona, northern California, Montana, Nevada, and Oregon, a miner’s inch equals 0.00071 m3/s.. |
| Mist irrigation | A method of microirrigation in which water is applied in very small droplets. |
| Mixed-flow pump | A centrifugal pump in which the pressure is developed partly by centrifugal force and partly by the lifting action of the impellers on the water. |
| Model | A simulation, by descriptive, statistical, or other means, of a process or project that is difficult or impossible to observe directly. |
| Moisture equivalent | Arbitrary soil water content used as an estimate for the field capacity of soils. It is the weight of water remaining in a soil sample after the soil has been saturated and subjected for 30 min to a centrifugal force 1,000 times gravity. |
| Mole drain | Drain formed by pulling a vertical blade and bullet-shaped cylinder through the soil. |
| Multi-stage pump | A pump having more than one impeller mounted on a single shaft. |
| N (index) | |
| Nappe | Sheet or curtain if water flowing from a structure, such as a weir or dam. |
| Natural flow | The flow of a stream as it would be if unaltered by upstream diversion, storage, import, export, or change in upstream consumptive use caused by development. |
| Natural system | (Drainage) System of drainage in which the main drains follows the largest natural depression from the outlet to the upper end of the area. |
| Navigable waters | Traditionally, waters sufficiently deep and wide for navigation by all, or specific sizes of vessels. |
| Net positive suction head, NPSH | The head that causes liquid to flow through the suction piping and enter the eye of the pump impeller. Required NPSH is a function of the pump design and varies with the capacity and speed of the pump. It must be supplied by the manufacturer. Available NPSH is a function of the system in which the pump operates and represents the energy level in the water over vapor pressure at the pump inlet. The available NPSH must equal or exceed the required NPSH or cavitation. |
| Nonpoint source pollution, NPS | Pollution originating from diffuse areas (land surface or atmosphere) having no well-defined source. |
| Nonpoint source pollution, NPS | Pollution originating from diffuse areas (land surface or atmosphere) having no well-defined source. |
| Non-renewable resources | Natural resources that can be used up completely or else used up to such a degree that it is economically impractical to obtain any more of them; e.g., coal, crude oil, metal ores. |
| Nonsaline-alkali soil | Soil containing sufficient exchangeable sodium to interfere with the growth of most crops. |
| Nonuniform flow | Flow in which the average cross sectional velocity is not the same at successive channel cross sections. If the velocity at a given cross section is constant with time, it is referred to a steady non-uniform flow. If the velocity changes with time at each cross section, it is known as unsteady non-uniform. |
| Normal depth | Depth of flow in an open channel during uniform flow for the given conditions. |
| No-tillage or no-till | A tillage system in which the soil is not tilled except during planting when a small slit is made in the soil for seed and agrochemical placement. Pest control is achieved through the use of pesticides, crop rotation, and biological control rather than tillage. Sometimes called “zero tillage” |
| Nozzle | Discharge opening or orifice of a sprinkler head used to control the volume of discharge, distribution pattern, and droplet size. |
| Nutrient | As a pollutant, any element or compound, such as phosphorus or nitrogen, that fuels abnormally high organic growth in aquatic ecosystems (e.g., eutrophication of a lake). |
| O (index) | |
| Observation well | Hole bored to a desired depth below the ground surface for observing the water table level. |
| Ogee | Profile of an overflow dam or spillway shaped in the form of an “S”. |
| Oligotrophic lake | Deep clear lakes with low nutrient supplies. They contain little organic matter and have a high dissolved oxygen level. |
| One-third atmosphere percentage | Water retained in an air dried and screened sample of soil that has been wetted and then brought to equilibrium on a permeable membrane at a soil moisture tension of 3.45 m of water, expressed as a percentage, or for field capacity in some soils |
| Open ditch outlet | Excavated open channel for disposing of drainage water from surface or subsurface drainage system, or for carrying flood water. |
| Operational efficiency | Ratio of the attained efficiency to the design efficiency. |
| Operational waste | Water that is lost or otherwise discarded from an irrigation system after having been diverted into it as part of normal operations. |
| Orchard valve | An outlet valve installed inside a pipeline riser with an adjustable cover or lid for flow control, similar to an alfalfa valve but with Lowe flow capacity. |
| Organic | (1) Referring to or derived from living organisms. (2) In chemistry, any compound containing carbon. |
| Organism | A living thing. |
| Orifice | An opening with a closed perimeter through which water flows. Certain shapes of orifices are calibrated for use in measuring flow rates. |
| Orographic storm | A weather pattern in which precipitation is caused by rising and cooling of air masses as they are forced upward by topography. |
| Outfall | Point where water flows from a conduit streams or drain. |
| Outlet | 1) An appurtenance to deliver water from a pipe system to the land, and individual sprinklers, lateral of sprinklers, or any surface pipe system. An outlet may consist of valve, a riser pipe, and/or and outlet gate. 2) Point of water disposal from a stream, river, lake, tidewater, artificial drain, terrace, or division. |
| Outlet demand | Channel constructed primarily to carry water from manmade structures such as terraces, subsurface drains, surface ditches, and diversions. |
| Outlet gate | A valve, usually a slide valve, that controls the flow of water from an outlet. |
| Overfall | Abrupt vertical change in stream channel elevation, the part of a dam or weir over which the water flows. |
| Overfall erosion | Erosion caused by falling down an overfall. |
| Overflow stand | Standpipe in which water rises and, at a given elevation overflows into a pipe or containment vessel. |
| Overhead irrigation | (See Sprinkler irrigation) |
| Overland flow | Surface runoff occurring at relatively shallow depths across the land surface prior to concentration in drainage ways. May cause sheet and rill erosion. |
| P (index) | |
| Pack material | Graded gravel and sand aggregates placed around a well screen or subsurface drain to prevent the infiltration of the fine materials. |
| Parallel drainage system | A drainage system with parallel laterals or field ditches that are perpendicular to the row drains. |
| Parshall flume | A calibrated device used to measure the flow of water in open channels, based on the principle of critical flow. |
| Particle-size analysis | Determination of the various amounts of the different separated in a soil sample, usually by sedimentation, sieving, or micrometry. |
| Parts per million (PPM) | The number of parts by weight of a substance per million parts of water. This unit is commonly used to represent pollutant concentrations. Large concentrations are expressed in percentages. |
| Pathogenic microorganisms | Microorganisms that can cause disease in other organisms or in humans, animals, and plants. |
| Peak use rate | Maximum rate of consumptive use of water by plants. |
| Percent area wetted | (Irrigation) Area wetted by irrigation as a percentage of the total crop area. |
| Perched water table | A water table, usually of limited area, maintained above larger ground water bodies by the presence of an intervening, relatively impervious confining stratum. |
| Percolating water | Subsurface water that flows through the soil profile of other porous media. |
| Percolation | The movement of water downward through the sub- surface to the zone of saturation. |
| Percolation rate | 1) The rate at which water moves through porous media, such as soil. 2) Intake rate used for designing wastewater absorption systems. |
| Perforated casing | A section of well casing with openings for water entry. |
| Perforated pipe | Pipe designed to discharge or accept water through small, multiple, closely spaced orifices, placed in its circumference. |
| Periglacial river | A river in the periglacial zone; i.e., the area adjacent to the margin of a glacier. |
| Permaculture | Integrated, evolving agricultural system of perennial or self-perpetuating plant and animal species. |
| Permafrost | Perennially frozen layer in the soil, found in alpine, arctic, and Antarctic regions. |
| Permanent wilting point | Soil water content below which plants cannot readily obtain water and permanently wilt. Sometimes called “permanent wilting percentage”. |
| Permeability | 1) (Qualitative) The waste with which gases, liquids, or plant roots penetrate or pass through a layer of soil or porous media. 2) (Qualitative) The specific soil property designating the rate at which gases and liquids can flow through the soil or porous media. |
| Permeameter | Device for containing the soil sample and subjecting it to fluid flow in order to measure permeability or hydraulic conductivity. |
| Permissible velocity | Highest water velocity in a channel or conduit that does no cause erosion. |
| Pesticide | A substance or mixture of substances intended for preventing, destroying, repelling, or mitigating any pest. Also, any substance or mixture of substances intended to regulate plant or leaf growth. Pesticides can accumulate in the food chain and/or contaminate the environment if misused. |
| pH | An expression of both acidity and alkalinity on a scale of 0-14, with 7 representing neutrality; numbers less than 7 indicate increasing acidity and numbers greater than 7 indicate increasing alkalinity. |
| Photosynthesis | The manufacture by plants of carbohydrates and oxygen from carbon dioxide and water in the presence of chlorophyll, using sunlight as an energy source. |
| Phreatic divide | Boundary between segments of a subsurface body of ground water that contributes to different stream systems. |
| Phytoplankton | Usually microscopic aquatic plants, sometimes consisting of only one cell. |
| Piezometer | Device for containing the soil sample and subjecting it to fluid flow in order to measure permeability or hydraulic conductivity. |
| Piezometer head | Combined elevation and pressure head as measured from a reference plane |
| Piezometric line or surface | Line or surface having equal piezometeric head. |
| Pipe drain | Any circular subsurface drain, including corrugated plastic tubing and concrete or clay tile. |
| Pipe spillway | A pipe drain for transporting water through an embankment. Sometimes called a “culvert” |
| Piping | Erosion of flow channels through soil by flowing water. |
| Pitting | 1) Shallow pits of suitable capacity and distributing retain precipitation on rangeland or pasture. 2) Formation of small cavities in a surface by corrosion of cavitation. |
| Plankton | Tiny plants and animals that live in water. |
| Plasticity | (soil) Property of a wet soil, which allows deformed without appreciable volume change of cracking. |
| Plunge poolk | Scout hole formed by falling water at the base off a vertical overfalls such as the head cut of a gully or downstream of a dam. |
| Point gauge | Sharp pointed rot attached to a graduated staff and often with a vernier scale for measuring water surface elevation. |
| Point row | A short crop row that forms an angle with another row at the field industry. |
| Point-source pollution | Pollution of ground or surface water supplies at well-defined locations. Discharges of untreated wastewater are common point sources of pollution. |
| Polychlorinated biphenyls (PCBs) | A group of chemicals found in industrial wastes. |
| Pond | A small natural body of standing fresh water filling a surface depression, usually smaller than a lake. |
| Porosity | 1) (Soil) The volume of pores in a soil sample divided by the combined volume of the pores and soil sample. 2) (Aquifer) The sum of the specific yield and the specific retention. |
| Porous trickle tubing | (Microirrigation) Tubing with a uniformly porous wall. The pores are small and ooze water under pressure. |
| Portable pipe | Irrigation pipe, which is or can be moved between irrigation sets, such as sprinkler or gated pipe. |
| Positive displacement pump | A pump that moves a fixed quantity of fluid with each stroke of rotation, such as a piston or gear pump. |
| Potential evapotranspiration | Rate at which water, if available, would be removed from soil and plant surfaces. |
| Power unit | Motor or engine used to drive a machine such as a pump/ |
| Precipitation | Water falling, in a liquid or solid state, from the atmosphere to a land or water surface. |
| Precipitation flow | Flow into and through porous media or soil by way of cracks, root holes, and other paths of low resistance rather than uniformly through the whole media. |
| Preplant irrigation | Irrigation applied prior to seeding. Sometimes called “preirrigation”. |
| Pressure head | The pressure energy in the liquid system as the equivalent height of a water column above a given datum. |
| Pressure rating | The estimated maximum internal pressure that can be continuously exerted in a pipe or container with a high degree of certainty that it will not fail. |
| Pressure regulator | 1) A pressure-decreasing device designed to maintain a constant pressure at an irrigation sprinkler regardless of elevation changes of the sprinkler. 2) A device used to maintain a constant, desired pressure on a pipeline. |
| Primer | Device attached to a pump to remove air and cause water to fill the pump through the suction pipe. |
| Priming chamber | Chamber attached to or built into the casing of a pump to retain water for automatic priming. |
| Propeller pump | A pump which develops most of its head by the lifting action of vanes on the water. |
| Puddled soil | A soil which has lost its granular structure and is in a deflocculated condition as a result of tillage, usually when it was in a wet plastic condition. |
| Puddling | Act of destroying soil structure, usually by disturbing or compacting the soil at high water content, thereby reducing porosity and permeability. |
| Pump column | The pipe through which water from well pumps is conveying to the ground surfaces. |
| Pump drainage | Drainage system in which pumps are used to lift water into an outlet. |
| Pump efficiency | Ratio of the waterpower produced by the pump, to the power delivered to the pump by the power unit. |
| Pump stand | A structure to convey water from a pump to the inlet of a pipeline. |
| Pump submergence | Vertical distance between surfaces of the water supply and the inlet of the pump. |
| Pumped well drain | Well drilled into an aquifer which is pumped to lower the water table. |
| Pumping plant or station | A complete installation of 1 or more pumps together with necessary appurtenances such as power units, pumps, screens, valves, motor controls, motor protection devices, fences, and shelters. |
| Q (index) | |
| Quick condition | Condition in which water flows through the soil material (upward or horizontally) with sufficient velocity to significantly reduces the bearing capacity of the material through a decrease in intergranular pressure. Sometimes called ‘quicksand”. |
| R (index) | |
| Race | Channel that leads water to or from a turbine or pump. |
| Rack | Screen of parallel bars placed in a channel to catch debris. Sometimes called a “bar screen”. |
| Radial Flow | 1) Flow from a source or to a sink along radial lines. 2) Direction of flow in a centrifugal pump. |
| Radial gate | A pivoted water control gate whose face is usually a circular arc with center of curvature at the pivot. Sometimes called a “taintor gate”. |
| Radial-flow pump | A centrifugal pump that uses diffuser vanes to transform the velocity head into pressure head. Commonly call a “turbine pump”. |
| Radius of influence | Maximum distance from a well at which drawdown is significant. |
| Rain | Water falling to earth in drops that have been condensed from moisture in the atmosphere. |
| Raindrop erosion | Soil detachment resulting from the impact of raindrops on the soil. |
| Rainfall frequency | Frequency of occurrence of a rainfall event whose intensity and duration can be expected to be equaled or exceeded. |
| Rainfall intensity | Rate of rainfall for any given time interval, usually expressed in units of depth per time. |
| Random drainage system | Surface or subsurface drainage system of irregular pattern used on depressional topography. |
| Rating curve | Graphic or tabular presentation of the discharge of or flow through a structure or channel section as a function of water stage or depth of low. Sometime called a “rating table”. |
| Rating flume | Structure placed in a channel to maintain a consisted regimen for the purpose of measuring the flow and developing a stage-discharge relation. |
| Reach | A length of a stream or channel with relativity constant characteristics. |
| Reasonable-use rule | A concept of water law in which a landowner is given the right to the reasonable use of water for domestic or similar needs. |
| Receiving waters | Distinct bodies of water, such as streams, lakes or estuaries, that receive runoff or wastewater discharges. |
| Recession curve | Descending portion of a stream flow or hydrograph. |
| Recharge | Process by which water is added to the zone of saturation to replenish an aquifer. |
| Recharge area | Land area over which water infiltrates and percolates downward to replenish an aquifer. For unconfined aquifers, the area is essentially the entire land surface overlaying the aquifer and for confined aquifers; the recharge area may be part of or unrelated to the overlaying area. |
| Rectangular weir | A channel structure having a rectangular flow notch. |
| Recurrence interval | Time period, in which a given hydrologic event can be expected to be equaled or exceeded, usually expressed in years. |
| Recyclable | Refers to such products as paper, glass, plastic, used oil, and metals that can be reprocessed instead of being disposed of as waste. |
| Reduced tillage | Any stream of tillage that leaves plant residues on the soil surface. |
| Reducer | Coupler or conduit section in which the outlet end is smaller than the inlet end. |
| Regime | Condition of a stream with respect to its rate of flow. |
| Relative humidity | Ratio of the amount of water present in the air to the amount required for saturation of the air at the same dry bulb temperature and harmometric pressure, expressed as a percentage. |
| Relief drain | A system of subsurface drain tiles or tubing installed within an area having a high water table, in order to lower the water table or maintain it at a given level. |
| Renewable resource | Natural resource (e.g., tree biomass, fresh water, fish) whose supply can essentially never be exhausted, usually because it is continuously produced. |
| Reservoir | Body of water, such as a natural or constructed lake, in which water is collected and stored for use. |
| Residual | The semisolid part of sewage and bacterial and settled or removed from the treated wastewater. |
| Resistance coefficient | A quantitative expression of hydraulic resistance exerted by a conduit boundary on fluid flow. Examples are “n, ”C’”, and “f” in the Manning, Chezy, and darcy-Weisbach equations for velocity of uniform flow. |
| Resource | A person, thing, or action needed for living or to improve the quality of life. |
| Resource management system | A combination of conversation practices and management identified by land and water uses that, when installed, will control soil losses and maintain acceptable water quality to permit sustained use. |
| Retard | Permeable dikes or barriers built into a stream channel approximately at right angles to the bank line to deflect the main current away from and stop erosion of the stream bank. (See Jetty.) |
| Retardance class | A characterization of vegetation with respect to its resistance to flow, used primarily for designing vegetated waterways. |
| Retention | Precipitation on an area that does not escape as runoff; the difference between total precipitation and total runoff. |
| Return flow | That portion of the water diverted from a stream which finds its way back to the stream channel, either as surface or subsurface flow. |
| Return period | The frequency of occurrence of a hydrologic event whose intensity and duration can be expected to be equaled or exceeded, usually expressed in years. |
| Return-flow system | A system of pipelines or ditches to collect and convey surface or subsurface runoff from an irrigated field for reuse. |
| Reuse system | A system designed to collect runoff from a surface irrigated field for reuse. (See Return-flow system.) |
| Reverse grade | A slope or grade on a field surface, crop row, or channel, that slopes in the direction opposite to the prevalent or desired grade. |
| Revetment | Facing of stone or other material, either permanent or temporary, placed along the edge of a body of water to stabilize the bank and to protect it from erosion. |
| Revolving screen | Trash screen or rack in the form of a cylinder or continuous belt revolved by water passing through it or other power source. |
| Rill | Small channels eroded into the soil surface by runoff which can be filled easily and removed by normal tillage. |
| Riparian | 1) Pertaining to the banks of a body of water, a riparian owner is one who owns the banks. 2) A riparian water right is the right to use and control water by virtue of ownership of the banks. |
| River | A natural stream of water of substantial volume. |
| River basin | A term used to designate the area drained by a river and its tributaries. |
| Riverside drain | Drain adjacent to a riverbed to a point downstream where water can be discharged above the mean high water level of the river. |
| Rock-fill dam | A dam composed of loose rock usually dumped in place; often with the upstream part constructed of hand-placed rock and faced with an impervious surface of concrete, timber or steel, or a thin core of impervious clayey materials. |
| Root zone | Depth of soil that plant roots readily penetrate and in which the predominant root activity occurs. |
| Roughness coefficient | (Preferred term is Resistance coefficient.) |
| Row crow | Crop planted in rows far enough apart to allow cultivation between rows during the growing season. |
| Row drain | A small drain constructed with a plow or similar tillage implement to provide drainage into field drains or field laterals. Sometimes locally called “plow drain”, “quarter drain”, “header ditch”, or “annual drain”. |
| Row grade | The slope in the direction of crop rows. |
| Runoff | The portion of precipitation, snow melt, or irrigation that flows over and through the soil, eventually making its way to surface water supplies. |
| Runoff coefficient | Ration of peak runoff rate to rainfall intensity. |
| Runoff duration | Elapsed time between the beginning and end of a runoff event. |
| Runoff rate | The rate at which water flows from a watershed. The peak value is often calculated by the rational runoff equation. |
| S (index) | |
| Safe well yield | Amount of ground water that can be withdrawn for an aquifer without degrading quality or reducing pumping level. |
| Saline soil | Nonsodic soil containing soluble salts in such quantities that they interfere with the growth of most crops. The electrical conductivity of the saturation extract is greater than 4mS/cm (0.01mho/in.), and the exchangeable-sodium-percentage is less than 15. |
| Saline-sodic soil | Soil containing sufficient exchangeable sodium to interfere with the growth of most crops and containing appreciable quantities of soluble salts. The exchangeable-sodium-percentage id greater than 15, and the electrical conductivity of the saturation extract is greater than 4 mS/cm (0.01 mho/in.). |
| Saltation | Soil movement by water or wind where particles skip or bounce along the streambed or soil surface. |
| Saltwater intrusion | The invasion of fresh surface or groundwater by salt water. |
| Sand | Soil particles ranging from 50 to 200 um in diameter. Soil material containing 85% or more particles in this size range. |
| Sand lens | Lenticular band of sand in distinctly sedimentary banded material. |
| Sanitary sewers | Underground pipes that carry off only domestic or industrial waste, not storm water. |
| Saturated flow | Flow of water through a porous material under saturated conditions. |
| Scour | To abrade or wear away; the wearing away of a channel or surface as in a flood by flowing water. |
| Screen | 1) (Wells) A manufactured well casing with precisely dimensioned and shaped openings. (Compare with Perforated casing.) 2) (Canals) A device used to clean surface water of debris. |
| Sediment | Fragmented organic or inorganic material derived from the weathering of soil, alluvial, and rock materials; removed by erosion and transported by water, wind, ice, and gravity. |
| Sediment basin | Pond at the upper end of a conveyance or reservoir for detaining particle-laden water for a sufficient length of time for deposition to occur. |
| Sediment load | Amount of sediment carried by running water or wind. |
| Sedimentation | Deposition of waterborne or windborne particles resulting from a decrease in transport capacity. |
| Seep collars | An annular plate of concrete or other impervious material placed on the outside surface of underground conduits to lengthen the flow path and thus impede seepage. |
| Seepage | The movement of water into and through the soil from unlined canals, ditches, and water storage facilities. |
| Seiche | A periodic oscillation, or standing wave, in an enclosed water body the physical dimensions of which determine how frequently the water level changes. |
| Semiarid climate | Climate characterized as neither entirely arid nor humid, but intermediate between the 2 conditions. A region is usually as semiarid when precipitation averages between 250mm (10 in.) and 500 mm (20 in.) per year. |
| Septage | The liquid and semisolid contents removed from a septic tank. |
| Septic system | An onsite waste disposal system designed to treat and dispose of domestic sewage. A typical septic system consists of a tank and a system of pipe drains or a pit for disposal of the liquid effluent that remains after digestion of the solids by bacteria in the tank. |
| Septic tank | Tank used to hold domestic wastes when a sewer line is not available to carry them to a treatment plant; part of a rural on- site sewage treatment system. |
| Settling basin | (See Sedimentation basin and Debris basin.) |
| Sewage | The waste and wastewater produced by residential and commercial establishments and discharged into sewers. |
| Sewage system | Pipelines or conduits, pumping stations, force mains, and all other structures, devices, and facilities used for collecting or conducting wastes to a point for treatment or disposal. |
| Sewer | A channel or conduit that carries wastewater and storm water runoff from the source to a treatment plant or receiving stream. |
| Sewer overflow | Flow of wastewater that exceeds the treatment capacity. It represents the flow that cannot be treated immediately and that is frequently discharged directly to a receiving stream without treatment, or to a holding basin for subsequent treatment and disposal. |
| Sewerage | The entire system of sewage collection, treatment, and disposal. |
| Sheet flow | Water, usually storm runoff, flowing in a thin layer over the soil or other smooth surface. |
| Shelterbelt | Extended windbreak of living trees and shrubs established and maintained for protection of farm lands or buildings. |
| Shingle | 1) Brush, paving, or cover placed with each overlapping layer extending farther upstream as in the manner of laying shingles. 2) A covering of stones, larger that gravel, developed on a beach or streambed. |
| Shutoff head | Pressure head on the outlet side of a pump at which the discharge drops to zero. Maximum pressure a pump will develop at a given speed. |
| Side inlet | (Drainage) Junction between a lateral and a main ditch of a surface drainage system. Side inlets are cut back level with the bottom of the main ditch for a distance to form a sediment basin. |
| Side slopes | Slope of the sides of a channel or embankment, horizontal to vertical distance (written 2:1). |
| Silt | 1) A soil separate consisting of particles between 2 and 50 um in diameter. 2) (Colloquial) Deposits of sediment which may contain soil particles of all sizes. |
| Siphon drain | Sealed drain where atmospheric pressure forces water over an intervening elevation into an outlet at a level lower than the inlet. |
| Siphon tube | Relatively short, light-weight, curved tube used to convey water over ditch banks to irrigate furrows or borders. |
| Skimming | Diverting surface water by means of a shallow overflow crest to avoid diverting sand, silt, or other debris carried as bed load. |
| Slick spot | Small areas of sodic soil, relatively impervious to water, that appear to be slick when wet, as a result of high exchangeable sodium content. |
| Slide gate | Head control valve, which slides on rails, used to control drainage or irrigation water. |
| Sludge | The solids which are removed from raw water or wastewater during water treatment. |
| Sluice | 1) Channel serving to drain off surplus water from behind a flood gate. 2) Conduit for carrying water at high velocity. 3) An opening in a structure for passing debris. |
| Snow course | A designated line along which the snow is sampled at appropriate times to determine its depth and density (water content) for forecasting water supplies. |
| Snow density | Water content of snow expressed as a percentage by volume. |
| Snow surveys | Measurements of the snow depth and density taken on regularly established snow courses at fixed schedules and used to forecast expected water supplies. |
| Sod strips | 1) Narrow bands of grass placed across a channel to spread and retard the flow of water. 2) Strips of sod maintained in a natural waterway to prevent erosion. |
| Sodic soil | A nonsaline soil containing sufficient exchangeable sodium to adversely affect crop production and soil structure. The exchangeable-sodium-percentage is greater than 15 and the electrical conductivity of the saturation extract is less than 4 mS/cm (0.01 mho/in.). |
| Sodium adsorption ratio, SAR | The proportion of soluble sodium ions in relation to the soluble calcium and magnesium ions in the soil water extract. |
| Sodium percentage | Percentage of total cations that sodium in water or soil solution. |
| Soil | The unconsolidated minerals and material on the immediate surface of the earth that serves as a natural medium for the growth of plants. |
| Soil aeration | Process by which air and other gases enter the soil or are exchanged. |
| Soil and water conservation district, SWCD | A local government entity within a defined water or soil protection area that provides assistance to residents in conserving natural resources, especially soil and water. |
| Soil compaction | Consolidation, reduction in porosity, and collapse of the structure of soil when subjected to surface loads. |
| Soil conservation | Protection of soil against physical loss by erosion and chemical deterioration by the application of management and land-use methods that safeguard the soil against all natural and human-induced factors. |
| Soil erodibility | A measure of the soil’s susceptibility to erosional processes. |
| Soil horizon | A layer of soil differing from adjacent genetically related layers in physical, chemical, and biological properties or characteristics. |
| Soil organic matter | Organic fraction of the soil, including plant and animal residues in various stages of decomposition, cells and tissues of soil organisms, and substances synthesized by the soil population. |
| Soil profile | Vertical section of the soil from the surface through all its horizons into the parent material. |
| Soil series | The lowest category of U.S. System of soil taxonomy. A conceptualized class of soil bodies having similar characteristics and arrangement in the soil profile. |
| Soil structure | The combination or arrangement of primary soil particles, into secondary particles, units, or peds that make up the soil mass. These secondary units may be, but usually are not, arranged in the profile in such a manner as to give a distinctive characteristic pattern. The principal types of soil structure are platy, prismatic, columnar, blocky or granular. |
| Soil texture | Classification of soil by the relative proportions of sand, silt, and clay present in the soil. |
| Soil water | All water stored in the soil. |
| Soil-water characteristic curve | Soil-specific relationship between the soil-water matrix potential and soil-water content. |
| Soil-water deficit or depletion | Amount of water required to raise the soil water content of root zone to field capacity. |
| Soil-water potential | The amount of work that must be done per unit quantity of pure water in order to transport reversibly and isothermally and infinitesimal quantity of water from a pool of pure water at a specified elevation at atmospheric pressure to the soil water at the point under consideration. |
| Soil-water pressure | The pressure (positive or negative), relative to the external gas pressure on the soil water, to which a solution identical in composition to the soil water must be subjected in order to be in equilibrium through a porous permeable wall with the soil water. |
| Solvent | Substances (usually liquid) capable of dissolving or dispersing one or more other substances. |
| Spearing | Technique of connecting crossing mole drains by forcing a spear or rod through both drain channels at the point of intersection. |
| Specific capacity | Well discharge divided by the water level drawdown after a specified pumping duration. |
| Specific retention | Amount of water that a unit volume or porous media or soil, after being saturated, will retain against the force of gravity. (Compare to Specific yield.) |
| Specific yield | Amount of water that a unit volume of porous media or soil, after being saturated, will yield when drained by gravity. (Compare to Specific retention.) |
| Spile | Conduit, made of lath, pipe, or hose, placed through ditch banks to transfer water from an irrigation ditch to a field. |
| Spillbox | Canal stabilizing structure. |
| Spillway | Conduit through or around a dam for the passage of excess water. May have controls. (See Emergency spillway.) |
| Spoil bank | Excavated soil piled along a canal, ditch, or basin. |
| Spoils | Dirt or rock that has been removed from its original location, destroying the composition of the soil in the process, as with strip- mining or dredging. |
| Spray irrigation | The application of water by a small spray or mist to the soil surface, where travel through the air becomes instrumental in the distribution of water. |
| Spring | An area where groundwater flows naturally onto the land surface. |
| Sprinkler distribution pattern | Water depth-distance relationship measured from a single sprinkler head. |
| Sprinkler head | A device for distributing water under pressure. |
| Sprinkler irrigation | Method of irrigation in which the water is sprayed, or sprinkled, through the air to the ground surface. |
| Sprinkler irrigation systems - Boom | An elevated, cantilevered sprinkler(s) mounted on a central stand. The sprinkler boom rotates about a central pivot. |
| Sprinkler irrigation systems - Center pivot | An automated irrigation system consisting of a sprinkler line rotating about a pivot point and supported by a number of self-propelled towers. The water is supplied at the pivot point and flows outward through the line supplying the individual outlets. |
| Sprinkler irrigation systems - Corner pivot | An additional span or other equipment attached to the end of a center pivot irrigation system that allows the overall radius to increase or decrease in relation to the field boundaries. |
| Sprinkler irrigation systems - Lateral move | An automated irrigation machine consisting of a sprinkler line supported by a number of self-propelled towers. The entire unit moves in a generally straight path and irrigates a basically rectangular area. Sometimes called a “linear move”. |
| Sprinkler irrigation systems - Permanent | Underground piping with risers and sprinklers. |
| Sprinkler irrigation systems - Portable(hand move) | Sprinkler system which is moved by uncoupling and picking up the pipes manually, requiring no special tools. |
| Sprinkler irrigation systems - Side-move sprinkler | A sprinkler system with the supply pipe supported on carriages and towing small diameter trailing pipelines, each fitted with several sprinkler heads. |
| Sprinkler irrigation systems - Side-roll sprinkler | The supply pipe is usually mounted on wheels with the pipe as the axle and where the system is moved across the field by rotating the pipeline by engine power. |
| Sprinkler irrigation systems - Solid set | System which covers the complete field with pipes and sprinklers in such a manner that all the field can be irrigated without moving any of the system. |
| Sprinkler irrigation systems - Towed sprinkler | System where lateral lines are mounted on wheels, sleds, or skids, and are pulled or towed in a direction approximately parallel to the lateral. |
| Sswamp | A type of wetland that is dominated by woody vegetation and does not accumulate appreciable peat deposits. Swamps may be fresh or salt water and tidal or non- tidal. |
| Stabilized grade | Slope of a channel at which neither erosion nor deposition occurs. |
| Stabilizer pack | Material placed around a well casing where the character of the aquifer does not require a filter as a formation and borehole stabilizer. |
| Staff gauge | Graduated scale, generally vertical, from which the water surface elevation may be read. |
| Stage | Elevation of a water surface above or below an established datum; gauge height. |
| Stand | Structure formed from vertical sections of pipe or from cast-in-place concrete (box stand). It may serve as a pump stand, gate stand, or float valve stand. It may also function as a vent or sand trap. |
| Stand gate | A valve in a structure which covers an inlet into or an outlet from a pipeline and controls water flow into or out of the pipeline. |
| Standby pump | Pump designed to operate only upon failure of a regular service pump. |
| Standing wave | Wave on the water surface at an acute angle to the flow which maintains a relatively constant position. |
| Standpipe | Vent constructed of vertical pipe on a pipeline, to relieve pressure surges and water hammer. |
| Static head | The potential energy due to elevation differences. (See Head.) |
| Static lift | Vertical distance between source and discharge water levels in a pump installation. |
| Steady flow | Open-channel flow in which the rate and cross-sectional area remain constant with time at a given station. |
| Stem flow | 1) Precipitation intercepted by vegetation that reaches the ground by flowing down the stems or trunks of vegetation. 2) Flow in the xylem of plants. |
| Stilling well | Pipe, chamber, or compartment, having closed sides and bottom except for a comparatively small inlet connected to a main body of water for attenuation of waves or surges while permitting the water level within the well to rise and fall with the major fluctuations of the main body. Used with water measuring devices to improve accuracy of measurement. |
| Stone drain | Underground channel with sides and top lined with flat stones, forming a generally rectangular or triangular section through which water passes. Differs from a blind drain in that there is a continuous conveyance channel. |
| Storage coefficient | (See Specific yield.) |
| Storage curve | Relationship between the volume of water stored and water surface elevation in a reservoir. |
| Storage efficiency or percentage | The ratio of the average depth of irrigation water infiltrated and stored in the root zone to the soil water deficit, expressed as a percentage. |
| Storm sewer | A system of pipes (separate from sanitary sewers) that carry only water runoff from building and land surfaces. |
| Stratified soils | Soils that are composed of layers usually varying in permeability and texture. |
| Stream | Any body of running water moving under gravity flow through clearly defined natural channels to progressively lower levels. |
| Stream bank stabilization | Vegetative or mechanical control of erodible stream banks, including measures to prevent stream banks from caving or sloughing such as lining banks with riprap, or matting and constructing jetties or revetments, as necessary, for permanent protection. |
| Stream channel erosion | Scouring of soil and the cutting of channel banks or beds by running water. Sometimes called “streambed erosion” or “stream bank erosion”. |
| Streamflow | The discharge that occurs in a natural channel. Although the term discharge can be applied to the flow of a canal, the word streamflow uniquely describes the discharge in a surface stream. The term streamflow is more general than the term runoff, as streamflow may be applied to discharge whether or not it is affected by diversion or regulation. |
| Stress irrigation | Management of irrigation water to apply less than enough water to satisfy the soil water deficiency in the entire root zone. (Preferred term is Limited irrigation.) |
| Strip cropping | Growing of different crops in alternating strips or bands along contours or across the prevailing wind direction to serve as vegetative barriers to wind and water erosion. |
| Stubble mulch | Protective over provided by leaving plant residues on the soul surface when preparing for and planting the next crop. |
| Subbing | 1) The process of a crop obtaining water directly from a shallow water table. 2) (Colloquial) The horizontal movement of water from an irrigation furrow to the row bed. |
| Subgrade | Earth material beneath a subsurface drain or foundation. |
| Subirrigation | Application of irrigation water below the ground surface by raising the water table to within or near the root zone. |
| Submerged aquatic vegetation | Aquatic vegetation (macrophytes) that cannot withstand excessive drying and therefore live with their leaves at or below the water surface. |
| Submerged flow | Flow through any critical depth measuring structure where the downstream water depth is high enough to interfere with establishment of critical velocity at the control section. Submergence is usually expressed as the ratio of downstream to upstream water level. |
| Subsoiling | Tillage operation to loosen the soil below the tillage zone without inversion and with a minimum of mixing with the tilled zone. |
| Subsurface drain | Subsurface conduits used primarily to remove subsurface water from soil. Classifications of subsurface drains include pipe drains, tile drains, and blind drains. |
| Subsurface drain storage | Volume of water that can be stored in the subsurface pipeline without reducing the effectiveness of the pipe or tile drain. |
| Subsurface drip irrigation | Application of water below the soil surface through emitters, with discharge rates generally in the same range as drip irrigation. This method of water application is different from and not to be confused with subirrigation where the root zone is irrigated by water table control. |
| Suction lift | Vertical distance between the elevation of the surface of the water source and the center of the pump impeller. |
| Suppressed weir | Measuring weir with sides and bottom flush with the channel, thus eliminating (suppressing) contractions or nappe of the overflowing water. |
| Surface collecting drains | Ditches used to remove pondages, and move water more rapidly into outlet drains. |
| Surface creep | Movement of coarse sediment in almost continuous contract with the soil surface during wind erosion. |
| Surface drainage | The diversion or orderly removal of excess water from the surface of land by means of improved natural or constructed channels, supplemented when necessary by shaping or grading of land surfaces to such channels. |
| Surface inlet | Structure for diverting surface water into an open ditch, subsurface drain, or pipeline. |
| Surface irrigation | Broad class of irrigation methods in which water is distributed over the soil surface by gravity flow. |
| Surface pipe outlet | Outlet for attaching a surface pipe to a riser without using a portable hydrant. |
| Surface retention | That portion of precipitation required to satisfy interpretation, the wetting of the soil surface, and depression storage. |
| Surface runoff | Precipitation, snow melt, or irrigation in excess of what can infiltrate and be stored in small surface depressions. |
| Surface sealing | Reorienting and packing of dispersed soil particles in the immediate surface layer of soil and clogging of surface pores resulting in reduces infiltration. |
| Surface soil | The uppermost part of the soil, ordinarily moved in tillage, or its equivalent in uncultivated soils, ranging in depth from 10 to 20 cm (4 to 8 in.). Sometimes called “soil management zone”. |
| Surface storage | Sum of detention and channel storage excluding depression storage, represents at any given moment, the total water enroute to an outlet from an area or watershed. |
| Surface water | Water flowing or stored on the earth’s surface. |
| Surge irrigation | A surface irrigation technique wherein flow is applied to furrows (or less commonly, borders) intermittently during a single irrigation set. |
| Survey cut | The difference between the initial and final survey gauge heights at a point during an excavation or land-grading operation. |
| Suspended sediment | Material moving in suspension in a fluid, due to the upward components of the turbulent currents or by colloidal suspension. Sometimes called “suspended load”. |
| Suspended solids (SS) | Defined in waste management, these are small particles of solid pollutants that resist separation by conventional methods. SS (along with BOD) is a measurement of water quality and an indicator of treatment plant efficiency. |
| Sustainable development | Development that ensures that the use of resources and the environment today does not restrict their use by future generations. |
| Swelling | (Soil) Physical expansion of the soil mass in an expanding type clay, usually caused by an increase in water content. |
| T (index) | |
| Tailwater | 1) Water, in a stream or canal, immediately downstream form a structure. 2) Excess irrigation water which reaches the lowers end of a field. |
| Temperature | The degree of hotness or coldness. |
| Tensionmeter | Instrument, consisting of a porous cup filled with water and connected to a manometer or vacuum gauge, used for measuring the soil-water matrix potential. |
| Terminal velocity | Final steady-state fall velocity of raindrops or sprinkler discharge drops. |
| Terrace | 1) A broad channel, bench, or embankment constructed across the slope to intercept runoff and detain or channel it to protected outlets. 2) A level plain, usually with a steep front, bordering a river, lake, or sea. |
| Terrace height | Difference in elevation between the bottom of the terrace channel and the top of the terrace ridge at a given cross section |
| Terrace inlet riser | Vertical pipe installed in a terrace channel and connected to an underground drainage pipe. |
| Terrace interval | Vertical or horizontal distance between 2 adjacent terraces. For the first terrace it is the distance between the top of the hill and the terrace. Depending on the interval under consideration, may be called “horizontal interval” and “vertical interval”. |
| Terrace outlet channel | Channel, usually having a vegetative cover, into which the flow from 1 or more terraces is discharged and conveyed from the field |
| Terrace System - Bench terrace | Level terraces built in stair-step fashion with a level to p and a steep, vertical embankment between successive terraces, used to improve distribution of rainfall or irrigation water. |
| Terrace System - Broadbase terrace | A type of terrace constructed so that corps a can be planted and machinery safely operated on the entire cross section. |
| Terrace System - Channel-type terrace | Terrace which is constructed by moving soil for the embankment from the uphill side only. |
| Terrace System - Conservation bench terrace | A broad level or flat channel terrace constructed below a sloping runoff, with erosion control as the primary objective. |
| Terrace System - Graded terrace | A terrace whose back and front slopes are constructed with steep but stable slopes and kept in permanent vegetation. |
| Terrace System - Level terrace | A terrace constructed along the contour with no slope and with either closed or open channel ends. Used primarily to retain runoff. |
| Terrace System - Mangum terrace | (Obsolete) Named for the originator of the broadbased terrace. |
| Terrace System - Narrow-base terrace | A terrace whose back and front slopes are constructed with steep but stable slopes and kept in permanent vegetation. |
| Terrace System - Nonparallel terraces | Terraces fitted to the contour of the land, which are not parallel to each other. |
| Terrace System - Parallel terraces | Terraces, not necessarily on the contour, aligned so that the adjacent terraces are nearly parallel to each other. |
| Terrace System - Ridgeless-channel terrace | A terrace constructed usually by excavation on nearly flat to gently sloping land to remove excess surface runoff water at nonerosive velocities. |
| Terrace System - Ridge-type terrace | Terrace embankment constructed usually by excavation on nearly flat to gently sloping land to remove excess surface runoff water at nonerosive velocities. |
| Terrace System - Steep-backslope terrace | A terrace constructed with the front slope and channel to be farmed and a steep but stable backslope, which is usually seeded to permanent grass. |
| Terrace width | Combined width of the terrace channel and ridge as measured horizontally from the upper edge of the channel to the edge of the lower slope of the terrace ridge. |
| Test hole | A bore hole drilled through underground formations to map the geology of the area or to evaluate the site as a potential well location. |
| Thermal pollution | The impairment of water quality through temperature increase; usually occurs as a result of industrial cooling water discharges. |
| Threshold velocity | The minimum velocity required to initiate movement of soil particles by direct pressure from water or wind. |
| Throat | Constricted flow area in a hydraulic structure. |
| Through fall | Precipitation reaching the ground beneath a vegetative canopy such as a forest; includes drip from the leaves and twigs, but not stem flow. |
| Tidal gate | Precipitation reaching the ground beneath a vegetative canopy such as a forest; includes drip from the leaves and twigs, but not stem flow. |
| Tile alignment | Degree to which the centerline of a tile falls in line with the centerline of adjacent lines. |
| Tile cradle | Support laid underneath a tile line in unstable soil to keep horizontal and vertical alignment of the tile line. |
| Tile density | Quality of a tile that determines its crushing strength, and its ability to resist water absorption and damage by freezing and thawing. |
| Tile drain | Drain constructed by laying drain tile with unsealed joints in the bottom of a trench which is then refilled. |
| Tile joint | Opening between 2 drain tiles through which water from the surrounding soil flows. (Compare with Crack width.) |
| Time or concentration | Time required for precipitation excess to flow from the most remote point of a watershed to the outlet |
| Toe drain | subsurface drain across the toe of an earth dam, designed to intercept water moving through the embankment. |
| Toe wall | downstream wall of a hydraulic structure. |
| Top width | horizontal distance across the top of a ditch or embankment. |
| Torpedo | channel forming head of mole plow. (Preferred term is Bullet.) |
| Total dynamic head | head required to pump water from its source to the point of discharge; equal to the static lift plus head losses in pipes and fittings plus the increase in velocity head. |
| Total suction head | head required to lift water from the water source to the centerline of the pump plus velocity head, entrance losses and friction losses in suction pipeline. |
| Toxic | Harmful to living organisms. |
| Trailing plug | plug following the mole plow torpedo, smoothing and strengthening the wall of the mole channel. (See Mole drain or Bullet.) |
| Transferable development rights | Compensation awarded to landowners whose use of land is restricted because of classification as prime farmland, floodplain, etc. |
| Transpiration | The process by which water absorbed by plants, usually through the roots, is evaporated into the atmosphere from the plant surface, principally from the leaves. |
| Trap | Enlargement in a conduit or hydraulic structure which reduces the flow velocity allowing sediment to be deposited. |
| Trapezoidal flume | A calibrated open-channel structure with sidewall inclined to the horizontal, used to measure the flow of water. Measurement is based on the principle of critical flow. |
| Trapezoidal weir | A sharp-crested weir of trapezoidal-shaped. (See Cipolletti weir.) |
| Trash rack | screen or gate at the intake of a channel, drain, or pump structure for the purpose of stopping debris. |
| Triangular weir | A sharp-sided 90 deg. V-notch weir. |
| Tributary | A stream that contributes its water to another stream or body of water. |
| Trickle irrigation | A method of microirrigation wherein water is applied to the soil surfaces as drops or mall streams through emitters. (Preferred term is Drip irrigation.) |
| Trickle spillway | A pipe or other conduit through an embankment to carry low flows to maintain a constant water level in a reservoir. |
| Tsunami | A Japanese term which has been adopted to describe a large seismically generated sea wave which is capable of considerable destruction in certain coastal areas, especially where sub- marine earthquakes occur. |
| Turbidity | Cloudiness caused by the presence of suspended solids in water; an indicator of water quality. |
| Turbine pump | A type of pump having 1 or more stages, each consisting of an impeller on a vertical shaft, surrounded by stationary and usually symmetrical guide vanes. Combines the energy-imparting characteristics of axial-flow and propeller pumps. |
| Turbulent flow | flow in which the fluid particles move in an irregular random manner, in which the head loss is approximately proportional to the second power of the velocity. |
| Turnout | (see Delivery box.) |
| Twin ditch | (See W-ditch.) |
| U (index) | |
| Unavailable soil water | that portion of water in a soil held so tightly by adhesion and other soil forces that it cannot be absorbed by plants rapidly enough to sustain growth. Soil water at permanent wilting point. |
| Unconfined aquifer | An aquifer whose upper boundary consists of relatively porous natural material that transmits water readily and does not confine water. The water level in the aquifer is the water table. |
| Undercutting | Erosion of material at the base of a steep slope, overfall, or cliff by falling water, stream, wind or wave action; produces an overhanging cliff. |
| Underflow | 1) Movement of water through a pervious subsurface stratum. 2) Flow of water under a structure or ice. |
| Underground outlet | A means of removing water from a terrace, consisting of an inlet riser and underground pipe that discharges water through or under the embankment. |
| Underground storage tank | A tank located all or partially underground that is designed to hold gasoline or other petroleum products or chemical solutions. |
| Uniform flow | Flow in which the velocity and depth are the same at each cross section. |
| Uniformity coefficient | 1) (Irrigation) A characterization of the aerial distribution of water in a field as the result of an irrigation. 2) (Soil) The ratio of the D60 size particle passing a screen to the D10 size of a granular material. |
| Unit stream | Amount of water, per unit of width, turned into each border strip or basin during irrigation. Sometimes called a “unit width stream”. |
| Unsaturated flow | Movement of water in soil in which the pores are not completely filled with water. |
| Unsaturated zone | That part of the soil profile in which the voids are not completely filled with water. |
| Urban runoff | Storm water from city streets and adjacent domestic or commercial properties that may carry pollutants of various kinds into the sewer systems and/or receiving waters. |
| V (index) | |
| Valve | A device to control flow. Valves used in pressurized systems include: |
| Valve - Air relief valve | Device that release air from a pipeline automatically without permitting loss of water. |
| Valve - Air vacuum, air relief valve | Device that releases air from a pipeline automatically without permitting loss of water or admits air automatically if the internal pressure becomes less than atmospheric. |
| Valve - Check valve | Valve used in a pipeline to allow flow in only one direction. |
| Valve - Drain valve | (a) Automatic: Spring-loaded valve that will automatically open and drain the line when the pressure drops to near zero. (b) Flushing type: Valve on the end of a line for the purpose of flushing out dirt and debris. This may be incorporated into an end plug or end cap. |
| Valve - Foot valve | Check valve used on the bottom of the suction pipe to retain the water in the pump when it is not in operation or prevent backflow. |
| Valve - Pressure relief valve | Spring-loaded valve set to open at pressure slightly above the operating pressure, used to relieve excessive pressure and surges. (See Air relief valve.) |
| Valve - Vacuum relief valve | Valve used to prevent a vacuum in pipelines and void collapsing of thin-wall pipe. |
| Vapor | The gaseous phase of substances that are liquid or solid at atmospheric temperature and pressure, e.g., steam. |
| Vapor pressure deficit | Difference between the existing vapor pressure and that of a saturated atmospheric vapor pressure at the same temperature. |
| Velocity head | Head or energy due to the velocity of a moving fluid; equal to the square of the mean velocity divided by twice the gravitational acceleration. |
| Vent | An appurtenance to a pipeline that permits the passage of air to or from the pipeline, |
| Venturi flume | Flow measuring flume with a contracted throat that causes a drop in the hydraulic grade line. (Preferred term is Parshall flume.) |
| Vertical drain | Vertical shaft to a permeable substratum into which surface and subsurface drainage water is channeled. |
| W (index) | |
| Wash load | That part of the sediment load of a stream that is composed of suspended clay and silt particles. |
| Waste disposal system | A system for the disposing of wastes, either by surface or underground methods; includes sewer systems, treatment works, and disposal wells. |
| Waste treatment lagoon | An impoundment for liquid and solid organic wastes designed to accomplish some degree of biochemical treatment of the wastes. |
| Wastewater | Water of reduced quality that has been used for some purpose and discarded. |
| Wastewater treatment plant | A facility containing a series of tanks, screens, filters, and other processes by which pollutants are removed from water. |
| Wasteway | Channel for conveying or discharging excess water from a canal to a river. |
| Water (H2O) | An odorless, tasteless, colorless liquid formed by a combination of hydrogen and oxygen; forms streams, lakes, and seas, and is a major constituent of all living matter. |
| Water amendment | 1) Fertilizer, herbicide, insecticide, or other material added to water for the enhancement of crop production. 2) A chemical water treatment to reduce emitter clogging. |
| Water application efficiency | Ratio of the average depth of water infiltrated and stored in the root zone to the average depth of water applied. |
| Water conservation | Protection and management of water resources for maximum sustained benefits. |
| Water contamination | Impairment of water quality to a degree which reduces the usability of the water for ordinary purposes, or which creates a hazard to public health through poisoning or spread of diseases. |
| Water conveyance efficiency | Ratio of the volume of irrigation water delivered by a distribution system to the water introduced into the system. |
| Water cushion | pool of water maintained to absorb the impact of water flowing from an overall structure. |
| Water harvesting | Any practice that enhances the runoff from a watershed or catchment area for collection and beneficial use. |
| Water holding capacity | Amount of soil water available to plants. (See Available soil water.) |
| Water leveling | A method of land grading wherein fields are divided into segments, flooded, and the highs are scraped down until all soil is beneath the water surface. Typically used with rice production. |
| Water management | The study, planning, monitoring and application of quantitative and qualitative control and development techniques for long- term, multiple use of the diverse forms of water resources. |
| Water pollution | Industrial and institutional wastes, and other harmful or objectionable material in sufficient quantities to result in a measurable degradation of the water quality. |
| Water quality | A term used to describe the chemical, physical, and biological characteristics of water with respect to its suitability for a particular use. |
| Water quality guidelines | Specific levels of water quality which, if reached, are expected to render a body of water suitable for its designated use. The criteria are based on specific levels of pollutants that would make the water harmful if used for drinking, swimming, farming, fish production, or industrial processes. |
| Water rights | Legal rights to use water supplies derived from common law, court decisions, or statutory enactments. |
| Water spreading | 1) Application of water to lands for the purpose of storing it as ground water for subsequent withdrawal. 2) A specialized form of surface irrigation accomplished by diverting flood runoff from natural channels or water courses and spreading the flow over relatively level areas. |
| Water storage efficiency | The ratio of the average depth of irrigation water infiltrated and stored in the root zone to the soil water deficit. |
| Water supply system | The collection, treatment, storage, and distribution of potable water from source to consumer. |
| Water table | The upper surface of a saturated zone below the soil surface where the water is at atmospheric pressure. |
| Water use efficiency | 1) Dry matter or harvested portion of crop produced per unit of water consumed. 2) Ration of water beneficially used to the water delivered to the area being irrigated. |
| Watershed | The land area that drains into a stream. |
| Watershed gradient | The average slope in a watershed measured along a path of water flow from a given point in the stream channel to the most remote point in the watershed. |
| W-ditch | Two-closely spaced, parallel single channels having the spoil from construction placed between them. To permit unimpeded runoff into them from surrounding lands. Sometimes called a “W-drain”. |
| Weir | 1) Structure across a stream to control or divert the flow. 2) Device for measuring the flow of water. Classification includes sharp-crested or broad-crested with rectangular, trapezoidal or triangular cross section. |
| Weir head | Vertical distance from the crest of a weir to the water surface in the forebay above the weir, not including the velocity head of approach. |
| Weir pond or box | Pond upstream from a weir usually used to reduce the velocity of approach and allow for full contraction of flow for measurement purposed. Also acts as a trap. |
| Well | A pit, hole, or shaft sunk into the earth to tap an underground source of water. |
| Well casing | Pipe installed within a borehole to prevent collapse of sidewall material, to receive and protect pump and pump column, and to allow water flow from the aquifer to pump intake. |
| Well development | The process of removing fine formation materials or materials introduced during well construction from the well intake zone for the purpose of stabilizing and increasing the permeability of the well intake zone and the filter pack material. |
| Well efficiency | Ratio of theoretical drawdown to measured drawdown. Theoretical drawdown is estimated from adjacent observation well data obtained during well test. |
| Well intake zone | The portion of the well surrounding the well inlet which is modified by the well construction and development processes. This includes the space between the well inlet and the undisturbed aquifer. |
| Well screen | That part of the well casing which has openings through which water enters. (See Screen and Perforated casing.) |
| Well test | Determination of the well yield versus drawdown relationship with time. |
| Well yield | Discharge rate that can be sustained from a well through some specified period of time. (See Safe well yield.) |
| Wet deposition | See acid rain. |
| Wetlands | Area of wet soil that is inundated or saturated under normal circumstances and would support a prevalence of hydrophytic plants. |
| Wetness index | Numerical quantity to designate the relation of precipitation or annual runoff for a given year to the long term average. |
| Wetted perimeter | Length of the wetted contact between a conveyed liquid and the open channel or closed conduit conveying it, measured in a plane at right angles to the direction of flow. |
| Wilting point | (Synonymous with Permanent wilting point.) |
| Wind erosion | Detachment, transportation, and deposition of soil by the action of wind. The removal and redeposition may be in more or less uniform layer or as localized blowouts and dunes. |
| Withdrawal use | The act of removing water from surface or groundwater sources in order to use it. |
| Z (index) | |
| Zone of saturation | A subsurface zone in which all the pores or the material are filled with groundwater under pressure greater than atmospheric pressure. |
| Zooplankton | Tiny aquatic animals eaten by fish. |