WSP09399 - Laserfiche WebLink

Home Browse Search

its system is designed to supp~y large str~ams to irrigators with minimum labor for application. This results iin large seepage losses which raises the groundwater table close to the surface,and the loss by evaporation increases rapidly. With the w~ter table 1 foot from the surface, the 1088 is almost equal to evaporation from a free .water surface. When water evaporates from soil, the salts are left and the. increased salinity may become destructive to; plants and soils. . Examples of deterIoration of irrigated areas owing to w~steful use of water with resulting rise of the water table are common ;through01,1t the world. . " . One reason fOr large waste of water is !thebeiief that best condi- tions for growth of plants occur when soil moisture is high. This belief is not founded upon facts. Greater growth is! not produced 1:ly frequent irrigations as compared to infrequent ones. As examples of such waste, cotton frequently is irrigated 8 to 10 times, and sugar 1:leets, tomatoes, alfalfa, and orchards much mo.re frequently than necessary. The soil is a reservoir. The amount of water thatca~ be held is limited by the kind of soil, and each soil has.a certain moiSture .content below which , water cannot be taken from it. Moisture is ~qually availabl'e to plants between the upper and lower limits, so plan~ growth is not increased by maintenance of large amounts of water in the soil. Transpiration is no greater at high soil moisture contents than a,t lower ones. Experiments show that for most deciduous orchards two or three irrigations are ample. For cotton, three applications usually will produce maximum yields. Alfalfa need not be irrigated more than once between cuttings on soils with high water-holding capacities. Water can be conserved if irri- gation is scheduled in accordance with the moisture properties of the soil and the root depth of the plants, and if the aim-ount applied is no more than enough to refill the soil reservoir when it i~ depleted. Most farmers apply the same amount of water at each irr~gation. If 40 to 50% is lost each time, considerable savings can be obta.ined by reducing the number of irrigations. The irrigated area in California could be almost doubled without increasing the demand for water if thiB wasted water were conserved. Some crops necessitate wasteful use of water. In California one \lrop of lettuce U!3es only a1:lout 4 inches of water, but frequently 3. to 4 feet are applied. Lettuce. plants. have sparse root systems, mostly in the surface soil, So that frequent water applications ~y be necessary. Since it is generally impractical to put on light applic~tibns 1:ly surface means, the total amount applied per crop is high. Irr~gated pastures also take large amounts of water, usually two or more times those for alfalfa. Although dome.stic and industrial. uses of water at present are small compared to agricultural use, savings canlpe effected. ~uch water i. used in cities for irrigation of lawns. Me~sureroents have. shown as much asZor.3 times more water is applied tha~actually required. For example, although most golf greens are irrigated eV,ery day, we have kept those at Davis green the entire year with two irriglttions. It takes persistence to get the granes to. establish a normal root1ngdepth, however.Evapora- tive coolers al.so cause waste of water. In over-all consideration of water c; 011 sumption, it should be keptinrnind that industdal and urban uses of water must go hand in hand with agricultural developments, Would it not be better to deny water to pOQr lands and use it for domestic and. industrial purpo.ses if the total supply! in a given area is not sufficient fo:t allpurposes? . . -16- . iIo . . . ..