Statement by Bureau of Reclamation on Hydrologic

Document: Statement by Bureau of Reclamation on Hydrologic Basis for Colorado River Water Supply Projections, July? 1965 (carbon copy), page 3With the Upper Basin reservoirs full at the start of the critical period, 1930-1964, which the hydrologic studies demonstrate would have been the case, instead of only an average annual dependable consumptive use of 4,300,000 acre-feet, the Upper Basin (again the Mexican Treaty is not here considered) could have realized an average annual dependable consumptive use of 6,200,000 acre-feet, derived as follows: average annual virgin flow at Lee Ferry, about 13,000,000 acre-feet, plus an average annual storage withdrawal of 700,000 acre-feet minus an average annual delivery at Lee Ferry of 7,500,000 acre-feet. Again we emphasize that the effectiveness of the Upper Basin reservoirs in regulating the flows of the Colorado River at Lee Ferry over the critical period required the entire period of record to be taken into account, not merely the flows during the critical period. It is the earlier high flows that provided the full reservoirs at the beginning of the critical period. In the Lower Basin, storage in Lake Mead is available to increase the yield of the river to the Lower Basin over the critical period. An operational pattern for Lake Mead similar to that for the Upper Basin reservoirs is used in our studies so that Lake Mead would be filled in high runoff periods and depleted during low runoff periods. Taking the past record of Colorado River flow at Lee Ferry, this results in Lake Mead being filled when the critical period began. In the case of power also, it is apparent that we have not always clearly and simply explained the underlying hydrologic concepts. Here, too, you will understand that the explantion which follows is designed to illustrate these concepts in a very simplified form, more or less avoiding complexities which will occur in actual practice. The capacity of the hydroelectric powerplants in kilowatts is determined by the size of the installed units, which in turn is related to the maximum depth of water in the reservoirs. However, the amount of marketable firm power is governed by the minimum depth of water in the reservoirs during the critical period and by the energy generation, which in turn is related to the amount of water flowing through the turbines. If there were no supplemental sources of energy supply and we had to depend upon the Colorado River flows alone, the firm electrical energy yield of the Government's Colorado River system hydroelectric plants would be determined on the same basis as we have determined the safe water yield. That is to say, the firm energy of the hydroelectric powerplants would be their kilowatt-hour capability over the critical period starting with full reservoirs at the beginning of that period. 3Epson Perfection 4870 Photo, 400 dpi, 8 bit, 3,239,588 byte