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< . . . 3. Mass Balance Analysis The yield of the basin above Lee Ferry was determined from a simple mass balance procedure. Although the method was computerized, the basic equation was the following: Yield .. [0 + S(l+B)] - Rm n I-s where 0 .. streamflow for the critical period S .. surface storage available B .. bank storage coefficient Rm - minimum release to the lower basin s - percent basin-wide shortage n .. number of years in the critical period. The CRSS model, as explained above and in Appendix II, was used to determine the quantity 5(1+B) for use in the mass balance analysis. Although the CRSS model could have been used to determine yield, it is an unwieldy tool for shortage and probability analyses and would have required considerable trial and error work at considerable expense. The values input to the mass balance program are the annual natural flows at Lees Ferry for 1906-1986 (see Hydrology section, above), the amount of storage available, bank storage coefficient, percent shortage and minimum release. The program provides output values for yield, defines the critical period and computes the probability of meeting various demands higher than the firm yield, given the input constraints. The critical period is determined by examining all possible average flows and their associated period up to 50 years, over the period of record, in conjunction with the input storage value. The storage value of 24.762 MAF determined from the eRSS data was based on a monthly operation. Since the mass balance procedure uses only annual data, it was necessary to make an adjustment to the storage value for use with the mass balance program. Adjustments were made for both the differences in the amount of streamflow over 25 years and seven months 6