- - AGRICULTURE CORE CURRICULUM - - (CLF7000) Advanced Core Cluster: PLANT AND SOIL SCIENCE (CLF7550) Unit Title: MANAGEMENT PRACTICES NECESSARY FOR IRRIGATION AND DRAINAGE ____________________________________________________________________________ (CLF7551) Topic: IRRIGATION AND Time Taught in Year(s) DRAINAGE 5 hours 3 and 4 ____________________________________________________________________________ Topic Objectives: Upon completion of this lesson the student will be able to: Learning Outcome #: (J-1) - Define the terms irrigation and drainage. (J-2) - List sources of irrigation water. (J-4) - Compare differences in water holding capacities and water infiltration rates of fine and coarse soils. (J-3) - Name and describe two common drainage systems. (J-10) - Describe factors that determine water penetration. Special Materials and Equipment: 3 small clay pots, 3 pint jars, 1 cup each of sand, silt, clay, 1 graduated cylinder (100 ml). References: Donahue, Roy L. SOILS. New Jersey. Pages 185-191, 489-507. Evaluation: Quiz by instructor TOPIC PRESENTATION: IRRIGATION AND DRAINAGE A. About three quarters of the earths surface is covered by water. 1. The problem is that less than 3% of that water is fresh water and we don't always have fresh water in the right place at the time we need it, and of the quality that is suitable for our needs. B. Irrigation is applying water to the soil, other than natural precipitation. C. Drainage is the removal of excess gravitational water from the soil by natural or artificial means. D. In addition to normal precipitation (usually snow or rain), the sources of water for irrigation are surface waters and groundwaters. 1. About 75 percent of the water used in the United States is surface water. In California, approximately 60 percent of the water used is surface water. a. Development for the use of surface water is usually cheaper and easier. b. The composition (amount of dissolved minerals) in surface water varies considerably. c. The composition of water is often critical to the uses that can be made of it. 2. Recently more use has been made of groundwater. About 20 percent of irrigation water used in the United States comes from well water. In California, about 40 percent of the irrigation water used comes from groundwater. a. This water exists, in underground reservoirs in the deeper soil and substratum. b. These underground reservoirs are called aquifers. c. A serious problem in using groundwaters is the rate that the aquifers will be recharged. d. Depending on the composition of the substratum, some aquifers will recharge in a few days; others may take years; some up to 3 to 4 thousand years. e. In some areas, as water is removed from underground aquifers and is not replaced, the surface land sinks (subsides). E. Two important factors that must be considered in the management of irrigation water are the water holding capacity and the water infiltration rate of our soil. 1. The water holding capacity of a soil is most directly related to the soil texture. a. A representative loam soil contains approximately 50 percent solid particles (sand, silt, clay and organic matter), 25 percent air, and 25 percent water. __________________________________ 25 % {| AIR |} {| _________________________________|} {| AVAILABLE WATER |} PORE SPACE 25 % {| - - - - - - - - - - - - - - - - -|} {| UNAVAILABLE WATER |} {| _________________________________|} {|//////////////////////////////////| 50 % {|///////////S O L I D S////////////| {|//////////////////////////////////| {|__________________________________| b. This representative loam will be comprised of approximately half solids and half pore spaces (voids). c. The pore spaces are occupied by half air and half water. d. For this loam soil, approximately one half of this water will be available to plant, and one half unavailable. e. When we compare lighter (sandier) soils to this loam we find that the water holding capacity decreases. f. Heavier soils will increase in water holding capacity. g. Typically the water holding capacities per foot of soil by textures are: 1) .5 to .75 inches of water per foot of sandy soil, 2) .75 to 1.25 inches of water per foot of loam soil, 3) 1.25 to 2.0 inches of water per foot of clay soil. h. The amount of soil compaction and the amount of organic matter in the soil will also affect its water holding capacity. 2. Water infiltration (water penetration) rates of a soil will vary with several factors. a. Water infiltration is rapid into large continuous pores in the soil; it is reduced by anything that decreases the size or amount of pore space. b. Infiltration rate is rapid in sand, slower in silt, and slowest in clay soils. c. Granular soil structure will increase infiltration rates. d. The greater amount of and the coarser the organic matter in the soil, the faster the water will enter the soil. e. The permeability of and the depth to hardpan, crusts, or other restricting layers will affect the rate of water infiltration. f. Wet soils do not have as high an infiltration rate as dry or moist soils. g. Compaction slows infiltration. h. Warm soils will absorb water faster than cold soils. Frozen soils may not be capable of absorbing water. F. About one third of the cropland in Canada and the United States is artificially drained. 1. Some of the benefits of drainage are as follows: a. Properly drained soils warm earlier in the spring. b. It makes the entire field more uniform in soil moisture. (Allows more predictable and efficient tillage.) c. It increases aerobic microbial activity. d. It reduces the amount of toxic substances (soluble salts, methane gas, sulfides) in the soil. e. Drained land is adapted to a wider choice of crops. f. Drained soil adds an additional volume of soil to the root zone (more available nutrients, thus a greater crop yield). g. Drainage is essential for reclaiming alkaline soils. 2. There are two common drainage systems, tiling and ditches. a. Tiling is a subsurface system of drainage. 1) A satisfactory layout for a tile drainage system requires much planning and experience. 2) When properly installed and maintained tile systems can last for more than 100 years. 3) Tile systems involve the laying of tiles (short sections of pipe made from fired clay or concrete) or continuous sections of perforated plastic pipe. 4) Gravel is usually poured around the tile to prevent entrance of soil particles into the drain. 5) Basically this is a system that interconnects and allows water to move underground and away from a parcel of land. b. Open ditches can also be used to drain land. 1) This is a surface system of drainage. 2) It is best adapted to relatively flat acreages. 3) Open ditches are designed to conform to the lands topography, land use, and soil characteristics. 4) The ditches are usually parallel and designed as a grid or herringbone pattern. 5) This system required a continuous maintenance of the open ditches (i.e., removal of soil, weed control). __________________________________________________________ ACTIVITY: 1. Demonstrate to the class the infiltration rate and water holding capacity characteristics of sand, silt and clay. (See details of this type of demonstration at the end of the Topic: "Irrigation Terminology". (CLF 352) Compare water holding capacity and speed of water penetration: Supplies needed: 3 pint jars; 3 small clay pots (bottom end will fit inside the pint jars) 1 cup of each of the following types of soil:sandy, silty and clay soils; 1 water measuring device (graduated cylinder or flask). Procedure: Place individual soil samples into each of the 3 clay pots. Place each clay pot into 1 of the pint jars. Place jars with pots in them in full view of the class, preferably in a row. Pour equal amounts of water (suggest 100 ml) into each of the 3 pots in succession (start with sand, then silt, and clay). After 15 minutes or when the dripping stops, measure the amount of water in each of the jars under the pots. (Use a graduated cylinder to reinforce the metric system and its uses in science.) Have the students complete a simple lab report on this activity. Include a description of the procedure, observations during the experiment and assessment of results (why). Observations: Water will saturate the sand rapidly and remaining water will flow through pot into jar. Silt will saturate a little more slowly. Clay will take the longest. Final results: Sands wet rapidly, but hold least water as observed in jar. Silt wets slightly slower, but holds more water. Heaviest soil wets slowest, but holds the most water. 2. Take the class on a fieldtrip to areas with ditch (surface) and/or tile (subsurface) drainage systems. __________________________________________________________ 7/25/90 sg #%&C