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<br />Reclamation, 1981). A similar resource study has been done for the South <br />Platte River basin (U.S. Army Corps of Engineers, 1977). The Arkansas River <br />basin has been described (Colorado Water Conservation Board and U.S. Soil <br />Conservation Service, no date), and a more recent description of water- <br />operating systems in the basin is available (Abbott, 1985). The water <br />resources of the Rio Grande basin have been described (Emery and others, <br />1973), and ground-water use in this area was examined in some detail in a <br />recent modeling study (Hearne and Dewey, 1988). <br /> <br /> <br />Localized studies that contain water-use information include ground- <br />water modeling studies, power-company studies of irrigation water demand <br />within their service area, public-supply-system reports on water deliveries, <br />and engineering-company studies of proposed water developments. In this <br />report, localized studies of interest will be referenced during discussion <br />of specific water-use topics. <br /> <br />Acknowledgments <br /> <br />Personnel at the State Engineer's Office provided access to their data <br />base as well as helpful discussions concerning water use and the administra- <br />tion of water resources in Colorado. The U.S. Environmental Protection Agency <br />and the Colorado State Department of Health also provided access to data <br />bases. Personnel at the U.S. Bureau of Reclamation were helpful in <br />discussions of methods for estimating consumptive water use in the Colorado <br />River basin. Individual water users throughout the State were helpful and <br />patient in answering inquiries as the authors became familiar with the many <br />ways in which water is used in this State. <br /> <br />WATER USE <br /> <br />A schematic of a water-use cycle is shown in figure 3. The cycle begins <br />when water is removed from the hydrologic system and ends when water is <br />returned to the hydrologic system. The water-use cycle is divided into three <br />parts: the source, use, and disposition of water. The water-use information <br />in this report was derived by aggregating all of the water-use cycles within <br />each county cataloging unit. County and hydrologic subregion totals then were <br />calculated by summing county cataloging unit data. For the purposes of this <br />report, it was assumed that water was withdrawn and returned within the same <br />county cataloging unit where it was used. Man-made conveyance structures <br />(pipelines, canals, tunnels) were considered to be part of the natural <br />hydrologic system. Withdrawal of water for conveyance or storage purposes <br />alone was not considered a use. It should be remembered, however, that humans <br />have a substantial effect on where water flows in Colorado. In 1985, 40 <br />structures existed for the interbasin transfer of water between hydrologic <br />subregions (fig. 4 and table 2). These structures conveyed a total of 751,000 <br />acre-feet of water during 1985. The volume of water transferred by some of <br />these structures can vary quite a bit from year to year; for instance, only <br />299 acre-feet of water was diverted through the Harold D. Roberts Tunnel <br />(map number 15 in fig. 4 and table 2) during 1985, while 134,000 acre-feet was <br />diverted during 1978 (Petsch, 1985). Some water is transferred more than <br />once. For instance, water used by the City of Aurora is diverted from the <br /> <br />7 <br />