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<br /> <br />3. Mass Balance Analysis <br /> <br /> <br />o <br />.} <br />W <br />~ <br />. O':l <br />...:.t <br /> <br />The yield of the basin above Lee Ferry was determined from a simple <br />balance 'procedure. Although the method was computerized, the basic <br />equation was the following: <br /> <br />[0 + 5(I+B)] - Rm <br /> <br />Yield <br /> <br />- <br /> <br />n <br /> <br />I-s <br /> <br />where 0 - streamflow for the critical period <br />S - surface storage available <br />B - bank storage coefficient <br />Rm . minimum release to the Lower Basin <br />s - percent basin-wide shortage <br />n - number of years in the critical period. <br /> <br />The eRSS model, as explained above and in Appendix II, was used to <br />determine the quantity S(l+B) for use in the mass balance analysis. <br />Although the eRSS model could have been used to determi~e yield, it is an <br />unwieldy tool for shortage and probability analyses and would have require <br />considerable trial and error work at considerable expense. <br /> <br />The values input to the mass balance program are the annual natural flows <br />at Lees Ferry for 1906-1986 (see Hydrology section, above), the amount of <br />storage available, bank storage coefficient, percent shortage and minimum <br />release. The program provides output values for yield, defines the <br />critical period and computes the probability of meeting various demands <br />higher than the firm yield, given the input constraints. <br /> <br />The critical period is determined by examining all possible average flows <br />and their associated period up to 50 years, over the period of record, in <br />conjunction with the input storage value. <br /> <br />The storage value of 24.762 MAF determined from the eRSS data was based on <br />a monthly operation. Since the mass balance procedure uses only annual <br />data, it was necessary to make an adjustment to the storage value for use <br />with the mass balance program. Adjustments were made for both the <br />differences in the amount of streamflow over 25 years and seven months <br /> <br />6 <br />