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Show -5- bandry these figures are reduced, and where water is plentiful and used lavishly they should be increased. It should also be understood that soil texture is a large factor in changing the above-mentioned percentage. It is, therefore, evident that 50 per cent or more of the water diverted at the river is available as a source to return flow. Underground Soil Reservoirs-Ordinary soil is porous, the voids ranging from 30 to 40 per cent. Each acre of saturated soil in irrigated areas will, therefore, reservoir 1 acre-ft. of underground water for each 3 ft. of depth. Unfilled underground soil reservoirs explain the heavy water consumption which occurs when irrigating new land and account for the lagging effect of return-flow waters. The magnitude of such underground reservoirs is illustrated by the Cache la Poudre in Colorado where 250,000 acres are irrigated. Assuming the average soil depth to bedrock (shale, sandstone and marls) to be 20 ft. and the average depth to the water table to be 5ft., there is a soil depth 7.5 ft. for underground water storage. Using a pore space of one-third, then 1 acre of surface soil would cover 5 acre-ft. of underground storage. In an area of 250,000 acres the underground reservoir would contain 1,250,000 acre-ft. The actual underground soil reservoir is more extensive because unirrigated areas lie within and adjacent to the boundaries of the irrigated tract. This means that during the 50 years of irrigation history in that valley there has been a gradual withdrawal of at least 1,500,000 acre-ft. of water from river circulation. This vast underground reservoir naturally fluctuates slightly from year to year, and in dry cycles is depleted, serving as an equalizer to river flow, but recovering during years of heavy stream flow and application. |
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Original book: [State of Arizona, complainant v. State of California, Palo Verde Irrigation District, Coachella Valley County Water District, Metropolitan Water District of Southern California, City of Los Angeles, California, City of San Diego, California, and County of San Diego, California, defendants, United States of America, State of Nevada, State of New Mexico, State of Utah, interveners] : California exhibits. |