AZ vs. CA:24 California Exhibits, Vol. XXIV; Miscellaneous

page 34

34 PKOBLEMS OF IMPERIAL VALLEY A*TD VICINITY. always been irrigated but has been gradually increasing to the present figure. This may be neglected. Item (d) will vary with the annual flow and must be very large in the high years. But as the years which determine the size of the reservoir are the low years, the error will be the minimum. A part of this land will be reclaimed and irrigated, and the floods, if Boulder Canyon is built, will cease to exist, so that a considerable error is introduced in the conservative direction by neglecting this indeterminate amount. The discharge at Boulder Canyon having been calculated, the next step estimates what it would be were this same cycle of years to recur after the development of the upper basin has been completed. The following items must be subtracted: (1) The average amount of water which has been consumed by increased irrigation above during the life of the record. (2) The average increase in diversions from the basin during the period. (3) The future estimated consumption of water which increased irrigation above will bring about. (4) The estimated increase in water diverted from the basin. (5) The increased evaporation from the surface of reservoirs in the canyon region for power regulation and from the, backwater caused by dams built to create power head. Increased development above-Items 1 and 2.-Various dependable estimates of irrigation above are as follows: 1902, 665,000 acres; 1915, 1,127,000 acres; 1920, 1,526,000 acres; increase, 1902 to 1920, 861,000 acres. A,large amount of this is in wild hay, the water consumption of which is small, but the increase in the lower Grand and Gunnison valleys was also large and here the land has been allpwed to become seeped, making a heavy consumption. It is assumed that the consumption has averaged 1 to 3 feet in depth per acre and that the increase in irrigable land has been gradual smce 1902. It is also assumed that this same gradual increase extended back to 1899. In 1902, diversions out of the basin were 7,000 acre-feet. In 1920, they were 127,000 acre-feet, an increase of 120,000. Placing these two items together, if the same cycle repeated itself, it is estimated that the mean annual discharge would be 730,000 acre-feet less than recorded. Future estimated consumption of water-Items 8 and 4--Future consumption of water for irrigation will vary from 1 foot in depth for wild hay to probably 2 feet in such areas as the San Juan in New Mexico and 2.5 feet in the warmer climate of the Virgin Basin. It is believed that the figures used for consumption in the following table are sufficient to include evaporation from local reservoirs which will be used for irrigation. They are not large enough to include an excessive evaporation from seeped lands. However, the upper basin, as a rule, has good natural drainage and it is assumed that if it becomes feasible to irrigate the expensive projects included in the list of ultimate acreage, drainage of any seeped lands which may exist will precede such development, since it would be less costly to reclaim such lands by drainage. The estimated depletion of the stream by development above considers storage in reservoirs and draft from them as compared to