Abstract Improving upon irrigarion water use efficiencies and adopting water conservation practices are receiving increasing attention as a solution to problems of inadequate water supplies. These methods are being pursued in lieu of more traditional methods of meeting growing water requirements through construction of more water supply facilities such as dams, conveyance facilities and wells. Since irrigated agriculture accounts for about 80 percent of all the water consumed and 50 percent of the total water diverted or withdrawn in the United States, it is assumed that by increasing irrigation water use efficiencies that substantial increases in the available water supply will result.
The Salt River Drainage Basin (Star Valley) as an agricultural watershed of 829 square miles in western Wyoming provided the opportunity for a study to determine the effects of increased irrigation efficiencies. Starting in 1971 several irrigation projects were completed that converted surface irrigation systems to sprinkler irrigation systems on approximately one-half of the irrigated acres in the valley. This conversion of irrigation systems resulted in less total water being diverted from streams for the sprinkler systems than was the case for the surface systems on the same irrigated acres.
Salt River stream flows were hydrologically analyzed and a comparison made of the flows prior to and after conversion to sprinkler systems. Significant impacts were identified. The mean monthly spring flows in the Salt River increased by 58.7 percent following the conversion to sprinkler irrigation. The study also showed substantially lower flows in the fall and early winter months. Analysis of annual flood peaks revealed that the mean annual flood peak flows increased by 47 percent.
Thus, this study shows that the primary effects of increasing irrigation efficiencies in areas where there is no storage above the irrigated area results in higher flows in the spring months, higher peak annual flow discharges and lower fall flows due to decreases in groundwater recharge. Large increases in spring flow also are causing bank erosion and damages of existing stream structures. The quantity of water available to the area essentially is unchanged but the time it is available has hanged substantially.
The study indicates that a careful analysis of resulting impacts within a watershed needs to be considered before major changes are made in the management of irrigation waters. Negative impacts upon the streamside zone land area and upon the quantity and quality of water may result
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