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. I . . . . . I . . . . . I . . I I - 0297 of the river are the direct result of recharge from applied irrigation water and precipitation. Prior to the practice of diverting surface water for irrigation, ground water levels were not high enough to maintain river flow throughout the year for the length of the river. In some areas the sand and gravel did not contain any water at all. With the application of surface water for irrigation, water began percolating into the alluvium beneath the fields and the water table rose. As a result of consumptive losses, due to evaporation and evapotranspiration, recharged ground water is higher in dissolved solids than the applied irrigation water. This creates a general increase in dissolved solids concentration in a down-gradient and down-valley direction within the alluvial aquifer. A long term water budget of streamflow shows the Arkansas River to be a gaining stream as a large part of the ground water recharge ultimately seeps into the river. Therefore the trend in surface water quality is an increase in dissolved solids concentrations in the river in a down-valley direction. IRRIGATION PRACTICES With the exception of dry land wheat and pasture, most crops in the area are irrigated. The main crops grown in the study area and their irrigated acreage is: 1. Hay 2. Corn 3. Sorghum Grain 4. WmterWheat 5. Vegetables 6. Dry Beans 7. Barley 8. Oats 122,400 54,600 20,300 19,700 8,600 4,200 1,200 1,000 The data for irrigated acreage in the study area by crop were based on 1992 agricultural statistics data from the Colorado Department of Agriculture. METEOROLOGY Based on U.S. Weather Bureau data, the climatic regime of the surrounding area would be classified as semi-arid. Mean annual precipitation in the area ranges from approximately 11 inches to 14 inches per year. Over 75 percent of the precipitation occurs during the period of April through September. Based on published information from the Colorado State Engineer's Office, the 100 year 24 hour precipitation event is approximately 4.5 inches and the 10 year 24 hour precipitation event is 3 inches. Normal annual Class A pan evaporation for this area is approximately 65 inches to 70 inches per year with the majority occurring during the May through October period. The general prevailing wind direction pattern is from the north and northeast.