The future health and economic welfare of the people and environment of Colorado depend on a continuous supply of fresh water. Detailed, comprehensive information on the use of water from Colorado's diverse surface-water and groundwater resources is important to water managers and planners by providing information they need to quantify current stresses and estimate and plan for future water needs. As part of the U.S. Geological Survey's (USGS) National Water Use Information Program (NWUIP), Statewide water withdrawal and water-use data have been collected or estimated and summarized in this report by county and by four-digit hydrologic unit code for the following seven water-use categories: irrigation (crop and golf course), public supply, self-supplied domestic, self-supplied industrial, livestock, mining, and thermoelectric power generation. A summary for instream water use for hydroelectric power generation also is included. This report is published in cooperation with the Colorado Water Conservation Board. In 2005, an estimated 13,581.22 million gallons per day (Mgal/d) was withdrawn from groundwater and surface-water sources in Colorado for the seven water-use categories. Withdrawals from surface water represented about 11,035 Mgal/d, or 81.3 percent of the total, whereas withdrawals from groundwater sources represented an estimated 2,546 Mgal/d or 18.7 percent of the total. Irrigation (combined crop and golf course) totaled 12,362.49 Mgal/d or 91 percent of the total water withdrawals in the State of Colorado. Crop irrigation accounted for 99.7 percent (12,321.85 Mgal/d) of the irrigation, whereas the 243 turf golf courses in Colorado accounted for 0.3 percent (40.64 Mgal/d) of the total irrigation water withdrawals. Total withdrawals for the other water-use categories were public supply, 864.17 Mgal/d; self-supplied domestic, 34.43 Mgal/d; self-supplied industrial, 142.44 Mgal/d; livestock, 33.06 Mgal/d; mining, 21.42 Mgal/d (includes both fresh and saline water); and thermoelectric, 123.21 Mgal/d. The counties with the largest total withdrawals (greater than 500 Mgal/d) were Mesa, Weld, Rio Grande, Montrose, Gunnison, and Saguache. Counties with the smallest total withdrawals (less than 5 Mgal/d) were Clear Creek, Gilpin, and San Juan. Four-digit hydrologic unit codes with the greatest withdrawals were 1019 (South Platte River Basin), 1301 (Rio Grande Basin), and 1102 (Arkansas River Basin); the high withdrawal rates were driven by crop irrigation withdrawals. Total instream water use for hydroelectric power generation was 5,253.60 Mgal/d. Groundwater withdrawals were estimated for 2004 for the bedrock and overlying alluvial aquifers in the Denver Basin for irrigation, public supply, commercial/industrial, household use only, and domestic/livestock water-use categories. Withdrawals were estimated for input into the USGS Denver Basin model by using the equations in the Senate Bill 96-074 groundwater model. The greatest withdrawals were for public supply. The smallest withdrawals were for household-use-only wells. Douglas County had the greatest groundwater withdrawals (183.98 Mgal/d), whereas Broomfield County had the smallest (3.09 Mgal/d). Of the seven Denver Basin aquifers, the Lower Arapahoe aquifer had the greatest total estimated withdrawals (287.11 Mgal/d), with Douglas County having the greatest public-supply withdrawal of any county (95.29 Mgal/d) from this aquifer. The Upper Dawson aquifer was the least used of the Denver Basin aquifers, based on estimated withdrawals of 17.64 Mgal/d. As part of the Colorado Statewide Water Supply Initiative (SWSI), forecasts of future water demand were made based on information such as population, climate, and then-current (2000) water-use information and did not include the effects of future water conservation. Categories compared between estimates in the SWSI baseline forecasted water demand and the USGS water-use compilation were limited to county population and w