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In this report, self-supplied withdrawals, deliveries from public suppliers (where applicable), and consumptive use estimates are given for seven categories of offstream use: domestic, commercial, irrigation, livestock, industrial, mining, and thermoelectric power. For the public-supply category, in addition to withdrawals, the report also gives water delivered to domestic, commercial, industrial, and thermoelectric power users. 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 consump- tive use of water during irrigation, the mineral content of the return flow often is substantially greater than that of the water applied. Consequently, irriga- tion 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 Instream use is a water use that takes place without the water being diverted or withdrawn from surface- or ground-water sources.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 SUppGlrt 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 hydro- electric 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. INTRODUCTION I 7 ^"{"~