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0724 <br />..� DESCRIPTION OF THE SYSTEM <br />The Arkansas River valley system modeled is a 150 -mile (240 -km) <br />reach from above Pueblo in southeastern Colorado to the Colorado - Kansas <br />State line. The system consists of the river, its tributaries, and 450 <br />square miles (1,200 km of alluvial aquifer in hydraulic connection <br />with the river. This valley, which is one of the major agricultural <br />areas of Colorado, produces alfalfa, corn, barley, sorghum, sugar beets, <br />and truck crops. Because the mean annual precipitation ranges from 12 <br />to 14 inches (300 to 360 mm), irrigation is necessary to raise.these <br />crops. <br />Water for irrigation is obtained from 23 canals diverting from the <br />river and about 1,400 wells diverting from the ground -water system. The <br />mean annual flow of the Arkansas River at Pueblo is 514,000 acre -feet <br />(6.3x10 8 m 3 ) and the mean annual flow of the tributaries between Pueblo <br />and the State line is about 320,000 acre -feet (3.9x10 m Canals were <br />constructed as early as the 1860's to divert the Arkansas River water <br />for irrigation. <br />River systems and canals in Colorado are operated under the Doctrine <br />of Prior Appropriation. Under this doctrine, the right to use the water <br />belongs to whomever first puts the water to beneficial use. A list of <br />appropriation dates and the associated right to divert a rate of flow is <br />maintained for each river system. This list determines how the available <br />water will be distributed among the various users. The first water right <br />in the system that was modeled has an appropriation date of April 1861. <br />Because of the Doctrine of Prior Appropriation, a change in water use on <br />a canal affects upstream users as well as downstream users. <br />6 <br />