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Limitations of Data Every person and most businesses in the State of Colorado use water. It is not possible to collect data for each of these millions of users. Some degree of estimation of use is inevitable. Within each water-use category there generally are a few large users and many small users. The strategy of the National Water-Use Information Program is to collect site-specific data for those large users that account for perhaps 80 percent of the water use in a category and to estimate the remainder. The resulting data base is there- fore a combination of site-specific information and estimated information. A long-term goal of the National Water-Use Information Program is to maximize the site-specific information in the data base and minimize the estimated information. No precise statements of accuracy of the water-use data can be made because the data were not acquired using rigorous statistical techniques and because very little comprehensive water-use data are available for comparison. In general, the data presented here vary in accuracy from one to three signi- ficant figures. For example, data about ground-water withdrawals for irrigation may be among the least accurate because they were calculated as a residual in an equation that relied on several assumptions. Data about withdrawals by public suppliers are more accurate because they consist primarily of site-specific information, combined with some estimates. Data about water use in power generation are among the most accurate because these data were acquired from a complete site-specific survey. The smallest reporting unit for this study was established at 0.01 million gallons per day (10,000 gallons per day). In areas where much site-specific water-use data are near this threshold value, rounding of data leads to some cumulative inaccuracy. Water-use information acquired from individual users often was reported as an annual quantity and had to be converted to a daily rate (million gallons per day) for this study. The use of this daily rate is not intended to imply that the actual rate of water use is uniform throughout the year. In fact, most water-use rates vary seasonally, especially within the domestic and irrigation water-use categories. Previous Studies Comprehensive water-use data for Colorado are scarce. In most cases, published data are limited to only a few categories of water use and include only water withdrawals or consumptive uses. The previously-mentioned reports by the U.S. Geological Survey provide State totals. An unpublished Draft State Water Plan discusses Colorado water use in some detail (Colorado Department of Natural Resources, 1981). River basin studies that include some data about water use have been published for each of the major basins of Colorado. The Colorado River basin has been examined in detail (Upper Colorado Region State-Federal Interagency Group, 1971), and the U.S. Bureau of Reclamation has estimated annual consumptive use in the basin since 1971 (see, for example, U.S. Bureau of 6