During 2005, withdrawals from ground-water and surface-water sources in South Dakota for the eight categories of offstream use totaled about 500 million gallons per day (Mgal/d). Of total withdrawals, about 271 Mgal/d was withdrawn from ground water and about 230 Mgal/d was withdrawn from surface water. The largest use of water in South Dakota during 2005 was irrigation, which accounted for about 58 percent of the total water withdrawn, followed by public supply, which accounted for about 20 percent of withdrawals. Public-supply systems served about 666,210 people, or about 86 percent of South Dakota's population in 2005. Public-supply systems withdrew about 100 Mgal/d in 2005. Ground-water withdrawals accounted for about 66 percent of the total withdrawals (66 Mgal/d), and surface-water withdrawals accounted for about 34 percent of total withdrawals (35 Mgal/d). Total public-supply withdrawals averaged about 151 gallons per day (gal/d) per capita. About 65 percent of the public-supply water was used for domestic purposes, and the average per capita domestic use was 99 gal/d. Self-supplied domestic withdrawals were about 8 Mgal/d, all of which was from ground water. About 109,750 people obtained household water from private wells in 2005, and per capita use was about 70 gal/d. Industrial self-supplied water use during 2005 was about 4 Mgal/d, of which about 98 percent was from ground water and about 2 percent was from surface water. Total withdrawals for thermoelectric use were about 5 Mgal/d, of which about 1 Mgal/d was from ground water and about 4 Mgal/d was from surface water. Total mining water use was about 10 Mgal/d, of which about 5 Mgal/d came from ground water and about 6 Mgal/d came from surface water. Total livestock water use was about 48 Mgal/d, of which about 19 Mgal/d came from ground water and about 28 Mgal/d came from surface water. Total aquaculture use was about 33 Mgal/d, of which about 19 Mgal/d came from ground water and about 14 Mgal/d came from surface water. Irrigation withdrawals during 2005 totaled about 292 Mgal/d, of which about 149 Mgal/d was from ground-water sources and about 143 Mgal/d was from surface-water sources. An estimated 421,830 acres was irrigated during 2005. Of the total acres irrigated, 298,160 acres was irrigated by sprinkler application and 123,670 acres was irrigated by surface (or flood) application. The only instream use reported for South Dakota was for hydroelectric power generation. During 2005, about 68,400 Mgal/d was used by the hydroelectric powerplants to generate about 3,688 gigawatt-hours of electricity. Total water use in South Dakota decreased by about 25 percent (175 Mgal/d) between 1985 and 2005 despite an increase in the State's population of about 70,000 people. Total ground-water use increased slightly (about 21 Mgal/d) between 1985 and 2005, whereas surface-water use decreased by about 195 Mgal/d. The decreases in both total use and surface-water use are mostly attributable to decreases in irrigation water use. Total irrigation water use decreased by about 168 Mgal/d between 1985 and 2005, and surface-water irrigation use decreased by about 204 Mgal/d. Ground-water irrigation use increased by about 36 Mgal/d between 1985 and 2005. Water use for public supply increased about 20 Mgal/d between 1985 and 2005, and the population served by public suppliers increased by about 118,000 people. In contrast, the number of people relying on private wells for domestic use decreased by about 48,000 between 1985 and 2005. All self-supplied domestic water use in 2005 was supplied by ground water. Total domestic use decreased about 8 Mgal/d between 1985 and 2005.