The High Plains is a major agricultural area, supported primarily by water from the High Plains aquifer, which is used to irrigate wheat and corn and to raise cattle and swine. The U.S. Geological Survey (USGS) and the Oklahoma Water Resources Board (OWRB) began a study of the High Plains aquifer in 1996. One purpose of the study was to develop a ground-water flow model that the OWRB could use to allocate the amount of water withdrawn from the a aquifer. The study area in Oklahoma covers all or parts of Beaver, Cimarron, Dewey, Ellis, Harper, Texas, and Woodward Counties. To provide appropriate hydrologic boundaries for the ground-water flow model, the study area was expanded to include parts of Colorado, Kansas, New Mexico, and Texas.