The High Plains aquifer underlies 174,000 square miles in parts of eight States (Colorado, Kansas, Nebraska, New Mexico, Oklahoma, South Dakota, Texas, and Wyoming). The aquifer is an important national resource, providing water for about 27 percent of the irrigated land in the United States and about 30 percent of the groundwater used for irrigation in the United States (Dennehy, 2000). Irrigation is the dominant water use in the High Plains, accounting for withdrawals during 1995 of more than 15 billion gallons per day (U.S. Geological Survey National Water Information System database). Substantial pumping of the High Plains aquifer for irrigation since about the 1940’s has resulted in water-level declines in some parts of the aquifer of more than 100 feet (McGuire and Sharpe, 1997). Concern about these declines led the U.S. Congress in 1984 to institute a water-level monitoring program for the aquifer. Water quality of the aquifer is a more recent concern. There have been local studies of water quality, but no large-scale, comprehensive assessment has been made of the entire aquifer system. Knowledge of the quality of water resources is important because of the implications to human and aquatic health. In 1991, the U.S. Geological Survey (USGS) began full implementation of the National Water-Quality Assessment (NAWQA) Program. The long-term goals of the NAWQA Program are to describe the status and trends in the quality of the Nation’s surface- and ground-water resources and determine the natural and anthropogenic factors affecting the water quality (Gilliom and others, 1995). The HPGW study began in October 1998 and represents a modification of the traditional NAWQA design in that the ground-water resource is the primary focus of the investigation. The HPGW study requires detailed and current information about the location of irrigated land for analyzing water-quality results with respect to land use and the selection of new study sites and for use in ground-water vulnerability modeling. The only existing information on irrigated land for the entire High Plains area is approximately 20 years old (Thelin and Heimes, 1987), and it only provides the percentage of irrigated land in 4-square-kilometer grid cells across the High Plains, not the actual locations of irrigated fields.
Acknowledgments: The timely effort of the U.S. Department of Agriculture’s (USDA) Farm Service Agency (FSA) in providing the historical ground-reference information for approximately 1,000 square miles of the High Plains is gratefully acknowledged. The authors also acknowledge the USGS National Mapping Discipline Earth Resources Observation Systems (EROS) Data Center for providing the original Landsat Thematic Mapper (TM) imagery.
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