Water in storage and approaches to ground-water management, High Plains aquifer, 2000

The High Plains (or Ogallala) aquifer under-lies a 111-million-acre area (173,000 square miles) in parts of eight States-Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming. The area that overlies the aquifer is characterized as varying "between a semiarid to arid environment and a moist subhumid environment" (Lohman, 1953) with gently sloping plains, fertile soil, abundant sunshine, few streams, and frequent winds. Though the area can receive a moderate amount of precipitation, precipitation in most of the area generally is inadequate to provide economically sufficient yield of typical crops-alfalfa, corn, cotton, sorghum, soybeans, and wheat. The 30-year average annual precipitation ranges from about 14 inches in the western part of the area to about 32 inches in the eastern part. The High Plains aquifer is generally composed of unconsolidated alluvial deposits. About 94 percent of the water pumped from the aquifer in 1995 was used for irrigation. Through irrigation of crops with ground water from the High Plains aquifer, the area overlying the aquifer has become one of the major agricultural producing regions of the world. In the area that overlies the High Plains aquifer, farmers began extensive use of ground water for irrigation in the 1940's. The estimated irrigated acreage increased rapidly from 1940 to 1980 and did not change greatly from 1980 to 1997: 1949-2.1 million acres, 1980-13.7 million acres, and 1997-13.9 million acres; irrigated acreage is estimated from farmer surveys or from satellite images (Heimes and Luckey, 1982; Thelin and Heimes, 1987; U.S. Department of Agriculture, 1999). The location of irrigated acres was estimated by vegetative conditions derived from Landsat satellite imagery and is presented as a percentage of a map cell that is 1-minute latitude by 1-minute longitude (Qi and others, 2002). During the satellite survey, greater than normal precipitation occurred in some areas. The greater than normal precipitation resulted in flooding in parts of the area and more viable crops in other parts of the area. Therefore, the amount of irrigated acreage probably is underestimated in some areas and overestimated in others (Qi and others, 2002). Annual pumpage from the High Plains aquifer for irrigation increased from 4 to 19 million acre-feet from 1949 to 1974; annual pumpage did not change greatly from 1974 to 1995 (Heimes and Luckey, 1982; USGS National Water-Data Storage and Retrieval System database). These pumpage totals were based on estimated and measured pumpage. Pumpage was estimated from crop consumptive irrigation requirements, which is the calculated minimum supplemental water required to maintain adequate soil water for optimal plant growth using factors such as crop type, soil, and weather patterns; from power usage; or from other data. Pumpage can be measured by flowmeters. The use of ground water from the High Plains aquifer resulted in substantial water-level declines in the aquifer. By 1980, more than 100 feet of water-level declines occurred in some parts of the aquifer, and by 1999 withdrawals resulted in more than 40 feet of additional water-level decline in larger areas of the aquifer (Luckey and others, 1981; McGuire, 2001). Studies that characterize the aquifer's available water and the water chemistry were conducted beginning in the early 1900's and are continuing to the present time. See "Selected Regional Studies of the High Plains Aquifer" for a brief description of some of these studies. Additional studies have been conducted in selected areas to estimate the effect of water-level declines and to evaluate methods to increase the usable water in the aquifer. See "Selected Local Studies of the High Plains Aquifer" for a brief description of some of these studies. This report describes the amount of drainable water in storage in the High Plains aquifer in 2000 and the changes in the amount of drainable water in storage in the aquifer from the time prior to significant ground-water development for irrigation (termed predevelopment in this report) to 2000. Drainable water in storage is the fraction of water in the aquifer that drains by gravity and generally can be withdrawn by wells. Remaining water in the aquifer is held to the aquifer material by capillary forces and generally cannot be withdrawn by wells. Drainable water in storage is termed "water in storage" in this report and was estimated using data collected from 1920 to 2000 from more than 20,000 wells screened in the High Plains aquifer. This report also summarizes approaches to ground-water management implemented by the States that overlie the High Plains aquifer. Estimates of water in storage depend on the accuracy and availability of the water-level altitude and base-of-aquifer data in all areas of the aquifer. In areas where few water-level data are currently available, such as southeastern Colorado, northwestern Nebraska, and parts of Wyoming, published maps supplemented available data. Map scale and density of water-level altitude and base-of-aquifer data preclude showing small areas where the saturated thickness or water-level change value might be more or less than indicated.