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's 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.

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Landsat multispectral-scanner data have been used to map irrigated cropland for determination of water use from the High Plains aquifer. Water-use estimates have provided one critical element in a groundwater flow model being developed by the U.S. Geological Survey. Information on irrigeited acreage and water use is needed to evaluate the effects of agricultural development on the High Plains aquifer. The High Plains aquifer is the primary source of water for one of the Nation's major agricultural areas covering about 174,000 square miles within parts of eight States. Several methods for determining irrigated acreage were evaluated. Digital analysis of Landsat data proved to be the most suitable approach and was used in a two-phase effort to map irrigated acreage for both the 1978 and 1980 growing seasons. The first phase, a test of analysis procedures, used 1978 Landsat data to map the majority of the High Plains. The test used a cluster-analysis technique to derive acreage estimates of irrigated cropland, nonirrigated cropland, and rangeland using 35 summer Landsat scenes. Based on the first-phase test results, several modifications were made to streamline and improve analysis techniques for the second-phase mapping of irrigated cropland using 1980 Landsat data. The analysis of 1980 data used a ratio technique to analyze the 59 spring and summer Landsat scenes required to provide acreage estimates for the major irrigated crops on the High Plains. Acreage estimates of irrigated cropland, nonirrigated cropland, and rangeland were aggregated to form a data base containing about 174,000 grid cells measuring 1 minute of latitude by 1 minute of longitude. Percentages for each land-use type were calculated and combined with sampled irrigation-application rates to compute estimates of irrigation water use for the groundwater flow model. An estimated 17,980,000 acre-feet of ground water was pumped from the High Plains aquifer during the 1980 growing season to irrigate 13,700,000 acres. To verify the reliability of the irrigated-acreage estimates used to calculate water use, an accuracy evaluation was conducted for the 1980 mapping of irrigated cropland using a multistage random-sampling method. The statistical evaluation confirmed that Landsat data and simple analysis techniques can provide an efficient tool for mapping irrigated cropland for the High Plains. However, availability of suitable Landsat scenes is required to provide a complete inventory of irrigated cropland. The techniques used to map irrigated cropland for the High Plains should be applicable to similar areas of the Western United States.

In addition, valuable information was provided by many other State and local agencies throughout the High Plains. Their contributions are an integral part of this investigation without which this study would not have been possible. The U.S. Geological Survey coordinated its investigation of the High Plains aquifer with a concurrent study by the Economic Development Administration of the Department of Commerce. The Six-State High Plains Ogallala Aquifer Area study conducted by the Economic Development Administration was authorized by Congress in 1976. This study was charged with the responsibility of examining the feasibility of increasing water supplies to insure the economic growth and vitality of the High Plains. Together, these two studies provide a comprehensive evaluation of the High Plains aquifer and the potential impacts of declining groundwater supplies on the region. The Economic Development Administration study has developed and proposed alternative strategies to alleviate or mitigate those impacts and the U.S. Geological Survey has provided hydrologic data and models needed to evaluate the effects of those strategies on the groundwater resource. CONVERSION FACTORS The following report uses inch-pound units as the primary system of measurements and metric units for water-chemistry measurements. The units commonly are abbreviated using the notations shown below in parentheses. Inch-pound units can be converted to metric units by multiplying by the factors given in the following list. Inch-pound unit Multiply by To obtain metric unit inch 2.540 X 101 millimeter foot (ft) 3.048 X 10'1 meter mile (mi) 1.609 kilometer acre 4.047 X 10" 1 hectare square mile (mi2) 2.59 square kilometer acre-foot (acre-ft) 1.233 X 10"3 cubic hectometer foot per day (ft/d) 3.048 X 10"1 meter per day cubic foot per second (ft3/s) 2.832 X 10~2 cubic meter per second

The study was charged with the responsibility of examining the feasibility of increasing water supplies to ensure the economic growth and vitality of the High Plains. Together, these two studies will provide a comprehensive evaluation of the High Plains aquifer and the potential impacts of declining groundwater supplies on the region. The Economic Development Administration study will develop and propose alternative strategies to alleviate or mitigate those impacts and the U.S. Geological Survey has provided hydrologic data and models needed to evaluate the effects of those strategies on the groundwater resource.