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~ --'"" -- ' ~ ~"-. 4 / INTRODUCTION ! ! category of water use. All the water-use information compiled for this report is stored in the U.S. Geological Survey's Aggregate Water-Use Data System (AWUDS). Sources of information and accuracy of data vary and are discussed for each category in subsequent parts of this report. More comprehensive analyses of field data and more detailed evalua- tions of existing water-use data were performed in the compilation of data for this report and for the 1985 water-use circular than for previous water-use circulars in this series. The increase in analyses and evaluations result from the U.S. Geological Survey's National Water-Use Information Program designed in 1978 to provide more uniform, current, and reliable information on water use. Documentation is available from each District Office that iden- tifies the sources of water-use information for that State and describes how the water-use estimates were determined for this report. As the State water- use information programs are developed and refined, the timeliness and accuracy of water-use data at the State and national levels will continue to Improve. Two regional meetings were held during 1990 with U.S. Geological Survey and State water-use personnel to familiarize them with available sources of water-use information and preferred methodo10,gies for data collection. Guidelines developed by the U.S. Geological Survey for prepara- tion of State water-use estimates were distributed at those meetings. The fol- lowing national data files were made available to each District for reference: U.S. Environmental Protection Agency Industrial Facilities Discharge files and Public Drinking Water Supply files, U.S. Bureau of Census population files, and the U.S, Department of Energy, Energy Information Administration reports. Each District was responsible for determining the most reliable source of information available for that State. Water-use numerical data are the average daily quantities used. Irriga- tion water is applied during only a part of each year, and at variable rates; therefore, the actual rate of application is much greater than the average daily rate given in tables in this report. In this report, the numerical data generally are rounded to three significant figures for values greater than 100 and two significant figures for values less than 100. Most tables show these data in million gallons per day. Selected tables also show per-capita-use data in gallons per day, rounded to three significant figures, and irrigation and hydro- electric power data in thousand-acre feet per year. A conversion table is given after the glossary to assist those readers who may wish to convert the data to other units of measurement. All numbers were rounded independent- ly; thus, the sums of individual rounded numbers may not equal the totals. The percentage changes discussed in the text, were calculated from the unrounded data. Population data, which are from the U.S. Bureau of the Census popula- tion estimates and projections (U.S. Bureau of the Census, 1991), are shown ~ ~