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of the data used in the analysis and much valuable advice and discussion on development of the statistical method. l\.:> o (::J OJ DATA AVAILABILITY At the beginning of this study, available data were reviewed to determine the type of analysis possible for estimating natural dissolved-solids dis- charge for water years 1906-83, which is the period used in operation of the CRSS model, Daily streamflow and periodic water-quality data for part of this period were available from the U.S. Geological Survey's National Water Data Storage and Retrieval System (WATSTORE) (Hutchinson, 1975); monthly streamflow and dissolved-solids discharge previously had been computed from these data (U.S. Department of the Interior, 1985; Liebermann and others, 1987). The periods of record for the monthly values are listed in table 1 for the sites included in this report. The U.S, Bureau of Reclamation provided estimates of annual irrigated area, monthly consumptive use by agriculture, and monthly natural streamflow from the CRSS data set for each site during 1906-83. The irrigated-area data had been compiled from several sources, including: State Engineer reports, census data, Colorado River Storage Project estimates, State agricultural statistics, Colorado Water Conservation Board reports, and others (U.S. Bureau of Reclamation, written commun., 1978). Because of inconsistencies in compu- tational methods, the irrigated-area data for 1976-83 was disregarded in the analysis for most of the sites. ,''; Agricultural consumptive use, as defined in the CRSS data set, is the difference between the volume of water diverted for irrigation and the volume of water that eventually returns to the stream, Therefore, consumptive use includes all evaporative losses during conveyance and application, crop transpiration, and phreatophyte transpiration along the canals and return-flow paths. It does not include evapotranspiration of natural precipitation. The monthly consumptive-use data were estimated by the U.S. Bureau of Reclamation using a modified version of the Blaney-Criddle formula (U.S. Soil Conservation Service, 1970). The areal extent of specific crops was estimated using county census data, ::-~ ""> -~ ' -.~ q 7 The U.S, Bureau of Reclamation computed natural streamflow as the sum of the historic streamflow and adjustments representing upstream development. Total adjustments included agricultural consumptive use, transbasin diver- sions, reservoir effects, municipal and industrial use, and incidental deple- tions. Monthly import and export data were obtained from the U.S. Geological Survey and irrigation districts. Reservoir effects consisted of evaporation and changes in surface and bank storage. Evaporation for each reservoir was computed by multiplying the monthly average water-surface area by the monthly rate of evaporation. Changes in bank storage were estimated as 10 percent of the change in surface storage. Monthly municipal and industrial use was estimated by u~iformly distributing annual powerplant consumptive use. Incidental depletions accounted for such losses as stock-pond evaporation and fish and wildlife use. For periods when historical streamflow data were unavailable, the natural-flow values were estimated using regression tech- niques (U.S. Bureau of Reclamation, written commun., 1983).