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<br />UIJIl.!15 <br /> <br />WATER SUPPLY <br /> <br /> <br />Surface Water <br /> <br />The total surface water supplies in the South Platte River Basin in <br />Colorado have been estimated to average 1.8 million acre-feet annually of <br />which abbut 336,000 acre-feet are attributed to transbasin imports. <br /> <br />The plains tributaries, such as Plum Creek, Cherry Creek, Box Elder <br />Creek, Kiowa Creek, Bijou Creek, etc. flow more in response to summer <br />rainstorms than from melting snow and therefore are not significant <br />contributors to the basin water supply. <br /> <br />As much as 75 percent of the runoff of mountain streams may occur during <br />the months of May through July as a consequence of melting mountain snowpacks. <br />Generally speaking, the mountain snowpack starts building in October and <br />continues until May. Low elevation snow begins melting in late April or early <br />May with the higher elevation melt continuing into early July. The pronounced <br />seasonal variations in streamflow explains the need for water storage <br />facilities found throughout the basin. <br /> <br />Transbasin Diversion projects such as Colorado-Big Thompson project, the <br />Moffat Collection system, etc. are important water supply sources for <br />agriculture, and municipal industrial purposes. <br /> <br />Natural surface water supplies of the South Platte Basin in Wyoming and <br />Colorado are shown in Table C-1. <br /> <br />By interstate compact, Colorado must let a specified amount of runoff <br />flow into Nebraska each year (if it is available under certain stipulations). <br />When imported water from other river basins is added to the basin native <br />supply and applied to the various uses, it is not all consumptively used <br />(especially during spring runoff). An estimate of annual water supplies and <br />depletions is shown in Table C-2. A 1977 report prepared for the U. S. Army <br />Corps of Engineers shows somewhat greater basin outflow than shown in Table <br />C-2, and concludes that about 300,000 acre-feet per year is still available <br />for Colorado use. The main reason this legally available water escapes from <br />Colorado is a lack of storage capacity to detain spring runoff and return <br />flows. <br /> <br />Existing reservoirs having individual capacities of 5,000 acre-feet or <br />more are listed in Table C-3. Many of these reservoirs are managed to provide <br />a basic irrigation water supply or to supplement direct streamflow diversions. <br />It is necessary to analyse reservoir stages and include this information along <br />with snow depths when forecasting the water supply each spring. Good <br />reservoir storage conditions can off-set to some extent a below average snow <br />depth situation within a basin. <br /> <br />C-I <br />