The High Plains aquifer, which underlies about 174,000 square miles in parts of eight States, is the principal source of water in one of the Nation\u27s major agricultural areas. About 170,000 wells pump water from the aquifer to irrigate about 13 million acres in the High Plains. In 1978, the U.S. Geological Survey began a regional study of the High Plains aquifer to provide geohydrologic data and computer models of the aquifer needed to evaluate the effects of ground-water development. This report describes the geohydrology of the High Plains aquifer. The High Plains aquifer consists mainly of hydraulically connected geologic units of late Tertiary or Quaternary age. The upper Tertiary rocks include part of the Brule Formation, Arikaree Group, and Ogallala Formation. The Quaternary deposits included in the aquifer consist of alluvial, dune-sand, and valley-fill deposits. The Ogallala Formation, which underlies 134,000 square miles, is the principal geologic unit in the High Plains aquifer. The Ogallala consists of a heterogeneous sequence of clays, silts, sands, and gravels deposited by streams that flowed eastward from the mountains. Within the Ogallala, zones cemented with calcium carbonate are resistant to weathering and form escarpments that typically mark the boundary of the High Plains. The High Plains aquifer is, regionally, a water-table aquifer consisting mostly of near-surface sand and gravel deposits. The maximum saturated thickness of the aquifer is about 1,000 feet and averages 200 feet. Hydraulic conductivity and specific yield of the aquifer depend on sediment types, which vary widely both horizontally and vertically. Consequently, hydraulic conductivity and specific yield also are highly variable. Hydraulic conductivity ranges from less than 25 to 300 feet per day and averages 60 feet per day. Specific yield ranges from less than 10 to 30 percent and averages about 15 percent. Ground-water flow generally is from west to east, at an average rate of about 1 foot per day, and discharges naturally to streams and springs, and directly to the atmosphere by evapotranspiration. Precipitation is the principal source of recharge to the High Plains aquifer. Estimated recharge rates range from 0.024 inch per year in part of Texas to 6 inches per year in south-central Kansas. Typically, recharge estimates are greatest for sandy soils. The High Plains aquifer contains about 3.25 billion acre-feet of drainable water. About 66 percent of the water in storage is in Nebraska and about 12 percent is in Texas. New Mexico, the State with the smallest water resource in the High Plains, has only 1.5 percent of the volume of water in storage
