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. . 5 2. Colorado River Simulation System (CRSS) Natural Flows - These are computed, unregulated. unaepleted flews at Lees Ferry. They are computed on a ~nthly basis takinl into conaideration Bureau and non-Bureau reservoirs, transbasin diversiona &ftG imports, munieipal and industrial uses, and depletions from con- sumptive uae eomputed by the modified Blaney-Criddle cathod. While monthly natural flava have been determined for 19 stations in the Upper Basin for use in the formula, only annual value. at Lees Ferry have been used. It is felt that these data may be somewhat more aeeurate than the Virgin Flows mentioned above. The formula. along with the two hydreJogy data set., were computerized in order to allow any combination of variable. and value. and their effects on alloweble depletions to be exam1ned. To valid~te the use of the formula in the present study, successful attempts to verify ~he previous (19b5) studie. were made. The.e ere discussed in the next section. After verification was obtained, run. using the new period of reeard were made to determine value. of allowable deple- tion values by use of the formula, and the value. were further verified by ~ using tne Colorado River Simulation Sy.tea (ClSS) computer model. &apresentative determination runs for the formula method and the model method are pre.ented in Tables 4 and 5 respectively. B. Discussi~n and Results An analysiS of a 1i=11ar study of Upper Basin depletion level by Tipton aD4 Kalmbach in July 1965, for the Upper Colorado River Co.m1..ion, revealed tbat they too used the Bureau's Virgin Flows a. a ba.is for the determination. In_ essence, they used a similar approach but solved the equation for U.eable Storage. They did this using two delivery assumptioUl - 7.5 KAt &cd 8.25 HAl, and varied the depletion level in each one. They then plotted Upper Ba.in depletion vs. required storage, With an a.aumed available .torage of 29.0 HAl, their plotted curves yielded a 5.6 ~~ depletion with a delivery of 8.25 MAl to the Lower Basin and 6.3 MAl with a delivery of 7.5 MAE. UaiOl the computerized formula with the Virgin Flows, zero shortage, 29.0 MAl of available storage and a 7.5 MAl delivery, an allowable depletion of 0.3 MAl was derived; with a deli- very of 8.25 MAE, the depletion was cocputed at 5.6 MAl thus verifying the Tipton and Kalmbach study. It must be noted. however, that the assumptions of 29.0 MAE available storage and a 7.5 MAE delivery co the Lower Ba.in used in the Tipton and Kalmbach study are not eonsistent with present Bureau operating poliCY. " I. Computerized Formula Results - Extension of Period Runs were stde using the computerized formula and each of the hydrology data sets with their re.pective full period. of record. Virgin Flows - !t.waa determined chat the addition of 16 years of data made no difference in the analysis diseua.ed above. Using this data set, the critieal period remains the same. Thus, the results obtained from the analysis re..in the same. --- CRSS Natural Flows - By fixin~ ~o of the variables to comply with present Bureau policy,(i.e., annual delivery to the Lower Basin of 8.25 MAE and allowable storage at 26.2 MAl), it waa ,determined that the allowable depletion in the Upper Basin would be 5.8 MAE. This is a conservative figure and is baaed upon the assumptions of no (zero) ahortage. and no additional water from bank storage release. U.ing the natural flows, the critical period .hifted to the