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<br />Pueblo Reservoir, near the upstream end of the study area, provides
<br />regulation of floodflows and storage of water for municipal and irrigation
<br />uses. It has a total capacity of about 358,000 acre-ft. During 1988, the
<br />storage volume ranged from a maximum of about 267,000 acre-ft in March to a
<br />minimum of about 149,000 acre-ft in October (U.S. Geological Survey, 1989 and
<br />1990).
<br />
<br />As part of the U.S. Bureau of Reclamation's Fryingpan-Arkansas project,
<br />Pueblo Reservoir provides supplemental irrigation water to 9 major and 2 minor,
<br />privately operated irrigation systems in the Southeastern Colorado Water
<br />Conservancy District. Collectively, these systems irrigate 240,000 acres
<br />and have an annual headgate diversion requirement of about 810,000 acre-ft.
<br />These systems had existed in the study area for about 100 years before
<br />receiving their first project water in 1978. Since 1978, annual water
<br />deliveries from the project have averaged about 23,000 acre-ft, or 2.8 percent
<br />of the required headgate diversions.
<br />
<br />In 1943, the U.S. Army Corps of Engineers began operating John Martin
<br />Reservoir to regulate floodflows and to apportion water in the Arkansas River
<br />between Colorado and Kansas according to guidelines of the Arkansas River
<br />Compact. In recent years, the reservoir also has been used to regulate
<br />irrigation-water supplies. John Martin Reservoir has a total capacity of
<br />615,000 acre-ft, of which about 270,000 acre-ft is reserved for flood control.
<br />During 1988, the storage volume ranged from a maximum of 310,000 acre-ft in
<br />April to a minimum of 79,000 acre-ft in October (U.S. Geological Survey, 1989
<br />and 1990).
<br />
<br />To supplement surface-water supplies, the alluvial aquifer, which is
<br />hydraulically connected to the Arkansas River, has been extensively developed.
<br />By the early 1970's, approximately 1,400 irrigation wells had been developed
<br />in the alluvial aquifer in Colorado (Taylor and Luckey, 1974), and by the
<br />early 1980's, about 450 irrigation wells had been developed in the alluvial
<br />aquifer in Kansas (Barker and others, 1983; Dunlap and others, 1985). The
<br />aquifer also is used for municipal supplies. The towns that use water from
<br />the alluvial aquifer either as a primary or a supplemental supply are Rocky
<br />Ford, La Junta, and Las Animas in Colorado, and Syracuse, Lakin, Deerfield,
<br />and Holcomb in Kansas.
<br />
<br />Runoff
<br />
<br />The primary source of streamflow in the Arkansas River basin is the
<br />snowpack that accumulates in high-altitude headwater areas during the winter
<br />months. Inflow to Pueblo Reservoir, near the western boundary of the study
<br />area, can be approximated by streamflow measured at Portland, Colo. Annual
<br />streamflow at this site for water years (October through September) 1940-88 is
<br />plotted in figure 5. From 1983 through 1987, streamflow waS much greater than
<br />the mean. During this period, reservoirs remained full, and releases were
<br />greater than normal. Runoff was less than normal during the year that samples
<br />were collected for this study (1988), but the hydrology and water quality
<br />within the study area were still affected by the large accumulation of runoff
<br />from the 5 previous years.
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