AZ vs. CA:17 California Exhibits, Vol. XVII; Nos. 2701-2724; 2800; 2901-2905; 3001-3050

page 13

-13- the period of 13,120,000 acre feet. It is often necessary to use such short periods of records, and many times even shorter periods on tributaries as well as at some points on main streams, since few or none are available for early years. To secure approximate records for longer periods, it is then necessary to extend these short period records by correlation with records at some point where the period covered is much longer. On the Upper Colorado River it is well known that the last twenty-five years cover a period containing a larger number of lower years than any other period of equal length on this river, there being three years in this period when the virgin flow was less than 8,000,000 acre feet. Planning reports based on this period will, therefore, be conservative if not ultra-conservative, as far as available water supply is concerned. If water is sufficiently valuable, the use of a limited number of years of short supply may be permitted in considering consumptive uses of water, and, hence, a somewhat larger available yield than 13,-000,000 acre feet may be assumed for certain purposes. Safe Yield of the Drainage Basin For general considerations, it is suggested that the safe yield of a stream in a given drainage area be selected SO' that its value will be exceeded from one-half to two-thirds of the time, depending upon the character of the stream flow and the purposes for which it is to be utilized. The final determination, of course, should depend upon more complete investigations utilizing annual flows, or, in some cases, monthly or daily flows for estimates of net safe yields. Considering the information available from these hydrographs, however, it can be shown that on the Colorado River at Lee Ferry an average flow of 13,-000,000 acre feet will be exceeded for forty years out of