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o ('''') w ..... U1 looM rate of nearly 5 percent. A large portion of tile population resides within Maricopa and Pima Counties, Arizona, and in Clark County, Nevada, Sixty percent of the Upper Basin and about 20 percent of the Lower Basin population were classified as rural with a significantly smaller per capita LIse of water. Both the urb'an and rural areas have the mutual problem of providing an adequate current and future water supply for a growing population in a wate'r-short area. As a result. of almost continuous studies concerning these problems, ,adequate production and effluent records are usually available to' adequately assess water use. Exports: Nearly all the transbasiil exports from tile Colorado Basin were measured and repprted by the Geological Survey, or local water commissioners and users. The remainder were, esti- mated on the baSis of past records and capacity of fa~ilities. Due to the high degree" of measurement, this area of basin consumptive use is considered to be quite accurat;ely determined. Lower Colorado River Mainstream The annual consumptive use of water from the Colorad9. River '. mainstream by the States and exports from the' system were taken from the Bureau annual report entitled "Compilation of Records in Accordance with Article V of the Decree, of the United States in Arizona v. California." The estimated Colo- rad9 River component' of the combined surface and subSurface return flows accruing to Las Vegas Wash and dis- charging into Lake Mead, as taken from the report, is credited. to Nevada's municipal and 1ndustrial water uses. Unmeasured subsurface return flows were estimated. below Hoover Dam, based partially on pr~liminary information supplied by the Task Force on Ground- Water Return Flows. All unmeasured subsurface return flows were credited to the irrigation water use taken from the Article V report, and were divided between California and Arizona based on their respective irrigation diversions. Gross evaporation from Lake Mead is estimated by the USGS and publiShed in its annual Water Resources Data reports. Net evaporation for Lake Mead is estimated by subtracting precipita- tion at nearby Boulder City, Nevada, from the gros s evaporat ion. Net evaporation from Lakes Mohave and Havasu and Senator Wash Reservoir was derived from available evaporation and preci~itation records and operating data. Annual channel losses were estimated as the inflow or outflow necessary to balance a simplified water budget for, the Lee Ferry to Hoover Dam and Hoover Dam to International Boundary reaches. Channel losses include evaporation, seepage, phreatophyte consumptive use, and bank storage. The accuracy of flow measurements. of the Colorado River mainst;ream for use in determining the channel losses values in table LC-l is in question. The gage error of an "excellent" USGS flow gage i,s 5 percent. Actual flow at Lee Ferry, Hoover Dam, or to Mexico may therefore vary approximately 400,000 acre-feet per year from values supplied by the USGS. Such inaccuracies, though a very small percent of total flow, will have dramatic effects on apparent channel loss computations. The annual land use, water supply, and water use information being gath- ered for the operation, maintenance, and administration of the Colorado River mainstream below Lee. Ferry is believed to be generally adequate in quantity, quality, and extent. These data are I,mder constant review and are being continually upgraded. Studies and programs are in progress to remedy a lack o.f data on return flows from mainstream_diversions. 15