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1929 DESCRIPTION OF STUDY AREA . ;-:nCl..ij\tIINAHl R'- ~C"" .2. ' ~"'f 'f"""\J.... ..e. f. . .....,Ud l::CT TO REVI~'r. - . . ..~. '.- The study area includes the Arkansas River Basin in southeastern Colorado from the foothills of the Rocky Mountains near Canon City to Lamar, a distance of about 240 mi (fig. I). The Arkansas River headwaters are located to the northwest of the study area near Leadville, Colorado. The river flows south and east through mountainous terrain. before emerging from the mountains near Canon C.itl', at an altitude of about 5.350 ft. At Pueblo. the river is impounded to form Pueblo Reservoir. Downstream from Pueblo, the river flows eastward across flat terraces and nearly level flood plains. an area commonly referred to as the lower Arkansas River valley. Immediately downstream from Las Animas. the river is impounded by John Martin Reservoir. About 58 mi downstream from John Manin Reservoir. the river flows into Kansas. The Arkansas River drains an area of about 25.400 mi~ in Colorado, includ- ing 4,669 mi2 upstream from Pueblo Reservoir. The semiarid climate of the study area is characterized by low to moderate precipitation, substantial evapora- tion, low humidity, moderate to intense winds, and a large daily range in temperature. Mean annual precipitation ~anges from 12 in. at Pueblo to 15 in. at Lamar. About 75 to 80 percent of the annual precipitation falls as rain during the growing season. ThroughoUlthe area, potential evapotranspiration greatly exceeds precipitation. Land use in the lower Arkansas River Basin is predominantly agricultural. Major crops are alfalfa, com, wheat, and sorghum. about 300,000 acres of which are irrigated. Most of the irrigated acreage is located in the alluvial valley of the Arkansas River, along the major tributaries, and near off-channel reservoirs. Crops types grown in the valley generally vary downstream by their salinity tolerance. Vegetables and other salt- sensitive crops are grown on proponionally more acreage upstream from La Junta, where salinity is lower: alfalfa, which is relatively salt tolerant. is grown on proponionally more acreage downstream from La Junta, where salinity is much higher 0.-!iles, 1977). The Arkansas River is a panially-penetrating stream that is incised into the alluvial deposits that form the valley-fill aquifer of the Arkansas River valley. The valley-fill aquifer, which extends from Pueblo to the down- .eam end of the study area, is an unconfined system which directly underlies and is in hydraulic connection with 9