Purpose and Scope
Sources of data and methods of analyses
This report discusses eight categories of offstream water use--- public supply, domestic, commercial, irrigation, livestock, industrial, mining, and thermoelectric power---and one category of instream use: hydroelectric power. Detailed information for other instream uses, such as navigation, recreation, pollution abatement, and fish habitat, is beyond the scope of this report. Information on wastewater-treatment facilities is given in the "Wastewater Release" section.
Information on many of the water-use categories in this report is more detailed than the information presented in previous water-use circulars in this series. For each category of offstream water use, 1990 withdrawal and consumptive-use estimates are discussed and those estimates are compared with corresponding 1985 estimates. The text is supplemented with illustrations and tables showing data for each State, Puerto Rico, the U.S. Virgin Islands, and the District of Columbia and for each of the 21 water-resources regions. (Water-resources regions are shown on a map on the inside of the front cover.) Totals are highlighted in the tables for ease of reference. At the beginning of this report is a section on total water use by category and source of water, and at the end is a section on trends in water use for the period 1950-90.
More comprehensive analyses of field data and more detailed evaluations 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 identifies 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 methodologies for data collection. Guidelines developed by the U.S. Geological Survey for preparation of State water-use estimates were distributed at those meetings. The following 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. Irrigation 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 hydroelectric 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 independently; 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 population estimates and projections (U.S. Bureau of the Census, 1991), are shown to the nearest thousand. Data on population served by public supply were compiled in cooperation with State and local agencies; these data are rounded to three significant figures.
Offstream use is a water use that depends on water being diverted or withdrawn from a surface- or ground-water source and conveyed to the place of use. To determine the total quantity of water used (self-supplied withdrawals and public-supply deliveries), five subtypes of use are evaluated, as explained below and shown in the following sketch.
Each category of use typically has different effects on the reuse potential of return flows. Reuse potential reflects the quality and the quantity of water available for subsequent use; for example, irrigation return flow may be contaminated by pesticides and fertilizers, and, because of the high consumptive use of water during irrigation, the mineral content of the return flow often is substantially greater than that of the water applied. Consequently, irrigation return flow frequently has little reuse potential. This is a significant contrast to the reuse potential of most water discharged from thermoelectric plants, where the principal change in the water is an increase in temperature.
Instream use is a water use that takes place without the water being diverted or withdrawn from surface- or ground-water sources. Examples of instream uses are hydroelectric power generation, navigation, freshwater dilution of saline estuaries, maintenance of minimum streamflow to support fish habitat, and the assimilation of wastewater.
Quantitative estimates for most instream uses are difficult to compile on a national scale. However, because such uses compete with offstream uses and affect the quality and quantity of water resources for all uses, effective water-resources management requires that methods and procedures be devised to enable instream uses to be assessed quantitatively.
The only instream-use estimates compiled for this report are for hydroelectric power generation. Unlike other instream uses, the water used for hydroelectric power generation is a measurable quantity because the amount of water passed through the plant can be documented. Consumptive use in actual hydroelectric power generation (as opposed to evaporation from impoundments created by hydroelectric dams) generally is negligible.
In this report, wastewater release refers to water released from private and public wastewater-treatment facilities. Information is provided on the number of publicly- and privately-owned wastewater-treatment facilities and on releases from only the public wastewater-treatment facilities. The releases can be either returned to the natural environment or reclaimed for beneficial uses, such as irrigation of golf courses and parks.
Last updated Monday, 24-Feb-2014 14:37:07 EST
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