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., -. .~ . Obtain review from Division Offices of the Division of Water Resources. . Obtain Colorado Water Conservation Board and other interested party review. . InCOI:pOIllte comments received. . Prepare a final report. 1.3 Background As an overview, the following discussion of Colorado's water supply is taken from the report "Water for Tomorrow, Colorado State Water Plan", (USBR, CWCB, February, 1974): The wtal water supply available to the state is derived through complex usage of supplies from several sources. Supplies pumped from deep and shallow aquifers usually have a close relationship with the surface water supply. The nonnal water supply situation including state water outflow and Transmountain diversions that prevailed in 1970 is summarized in Table 1.1. (In order to update Table 1.1 from 1974, it required many assumptions to be made. Updated information was supplied where readily available, however, it highlights the need for updated data to complete an accurate water budget for the state. The most difficult part of Table 1.1 to determine is the water depletions portion that requires significant data on diversions, consumptive use and return flows. This component of the water budget is discussed in Section 3.0). The annual surface runoff in any panicukzr stream varies widely from year to year depending upon the precipitation. Other factors that influence the runoff are the soils, topography, geology, and vegetative cover. The average annual runoff ranges from 20 inches in the high mountains to less than 0.25 inches in the arid pans of the plateau and plains sectors of the state. By sub basins, the Upper Colorado sub basin experiences the highest runoff, the average annual being 6.6 inches. The Republican River sub basin has an average annual runoff of 0.4 inches which is the lowest in the state. The water depletions in the state occur as a result of the utilization of a complex surface and ground water resource system. Some uses such as the generation of hydroelectric puwer cause little, if any, depletions. Also, in moSt cases water depletions for fish and wildlife, and recreational uses are minor; lwwever, there can be substantial non beneficial evaporation losses from reservoirs kept full for these uses. Water supplies used for irrigation result in much greater depletions than any other purpose or combination of purposes. However, substantial retum flows result from irrigation which are in tum put to successive uses. Other uses which result in lesser but substantial water depletion are mwzicipal and industrial use, and mining and processing of minerdls. 1 Interstate compacts require Colorado to permit specified quantities of water to cross its boundaries into other states for downstream use. In addition to the outflows required to satisfy 6