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<br /> <br />1 <br /> <br />. <br /> <br />~~ V.";"'t---4.-J,?5; <br />~ +-<.--\...... lc'q.C <br />1"1 I <br />~'~Nf.~t <br />M~ \r- <br /><; d "".:t: (1..-r..) <br />1MJ.~):J <br /> <br />Development of an Accounting System for Water and Consumptive Use in the Lower <br />Colorado River, Lake Mead to Mexico <br /> <br />. <br /> <br />United States Geological Survey <br />Colorado River Basin Salinity Control Program <br />Accomplishments for Fiscal Year 1994 <br /> <br />Arizona <br /> <br />In the United States, accounting for the use and distribution of Colorado River water in the <br />lower basin is required by the U.S. Supreme Court decree, 1964, Arizona v. California. The <br />u.s. Geological Survey, in cooperation with the Bureau of Reclamation, developed an <br />accounting system for estimating and distributing consumptive use of water by vegetation <br />among water users along the Colorado River from Hoover Dam to Mexico to enable the <br />Secretary of the Interior to meet legal responsibilities stated in the decree. The accounting <br />system is based on a water-budget method to estimate total consumptive use by vegetation <br />from Hoover Dam to MoreJos Dam. Consumptive use by vegetation is apportioned to <br />agricultural us~rs by using percentages of total evapotranspiration by vegetation estimated for <br />each diverter of water. Evapotranspiration for each diverter is estimated from (1) digital- <br />image analysis of data from the Landsat satellite to determine vegetation types and areas for <br />each diverter and (2) water-use rates to determine the quantity of water used by each <br />vegetation type. Evapotranspiration is estimated for each of four reaches of the river--Hoover <br />Dam to Davis Dam, Davis Dam to Parker Dam, Parker Dam to Imperial Dam, and Imperial <br />Dam to Morelos Dam--to incorporate spatial variations in the weather data used to calculate <br />water-use rates. <br /> <br />The accounting system produced reliable (less than one percent difference from the previous <br />method) results for 1984 when, because of an unusually large quantity of flow in the river, the <br />computed consumptive use by vegetation was less precise than anticipated. On the basis of the <br />analysis for 1984, the accounting system should yield accurate estimates of consumptive use by <br />agricultural users for all years. To improve the estimate of consumptive use by vegetation, <br />errors in computed flow at the mainstream gages should be further reduced. More accurate <br />computation of discharge at the major dams along the Colorado River will also facilitate the <br />use of water budgets for sub-reaches of the river to refine the estimates of consumptive use by <br />vegetation along the river. Water-use rates for vegetation types that more accurately reflect <br />spatial and temporal variability of evapotranspiration need to be developed to improve the <br />distribution of consumptive use by vegetation; the identification of minor crops and multiple- <br />cropped areas also need to be improved. <br /> <br />Project is complete. Ten publications are available and a Water Supply Paper, approved in <br />FY93, should be published in FY9S. <br /> <br />1377 <br /> <br />11 <br /> <br /> <br />