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00lG87 The CRSM currently uses a critical period of 12 years, from 1953 throu9h 1964. This critic~l period was derived using the following mass balance equation: 602(a) storage = (5.8 MAF depletions x n x 0.9388) + (8.23 MAF objective minimU'l1 releasl~ x n) _ (natural flow at Lees Ferry over the critical period of "n" years) where n = the length of the critical period, and 0.9388 provides for a shortage of 6.12 percent. Values of 602(a) storage were computed with the above equ,ltion using all possible s,~q'Jential hydrology periods of all possible lengths (n) in the 1906-1983 natur'al floill record. The hydrology period that gave the largest 602(a) storage was the 12-year period beginning in 1953. The CRSM computes 602(a) storage at the beginning of each calendar year usin9 the above mass balance equation with the following modifi- cations: (1) the depletions are equal to the depletions scheduled in the demand data file for the next 12 years being modeled, (2) the value of "n" is equal to 12, ann (1) the natural flow at Lees Ferry over the critical period is equal to 12.18 MAF. Note that the method of determining 602(a) storage described here is used for simulation only and is not intended to be a formal deter- mination of 602(a) storage ~s required by the Criteria for Long-Range Operati on of Colorado Ri ver ~,~s~rv()i rs. 8.3 Releases from_ !-_a_ke M~~<! Initial releases from Lake Mead are determined on a monthly basis by combi ni ng three factors: (1) schedul ed consumptive use requi rements dOillnstream of Hoover Dam, (2) the volume of water required to meet reservoi r target storage curves estdbl i sh,~d for Lake Mohave and Lake Havasu, and (3) the net volume of gains ~nd loss'~s for the month for all reaches of the system downstre~m of Hoover Dam. The process of deter- mining initial releases from Lake Mead is simply an accounting proce- dure, with all the data supplied externally to the simulation model. Initial releases from Lake Mead are used unless they are increased because of flood control or scheduled surpluses, or they are decreased because of shortage conditions. Flood control requirements are discussed in section 8.3.1. The surplus strategy is disclJssed in sec- tion 8.4 and the shortage strategy is discussed in section 8.5. 8.3.1 Flood Control Flood control operation of L~ke Mead is one of two factors that can increase releases from Lake Mead above those which are needed to meet downstream demands and target content level s of downstream reser- voirs. The CRSM simulates the flood control operation of Lake Mead 37