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<br />OfJ17J:: <br /> <br />storage values for Lake Mead in months August throu9h December are the <br />target contents needed to obtain required flood control space in Lake <br />Mead. The target values are computed by taking the maximum capacity of <br />Lake Mead, subtracting the requi red flood control space, and adding cre- <br />ditable flood control space in the Upper Basin reservoirs. <br /> <br />11.3 Shortages <br /> <br />The CRSM computes the amount of shortage that occurs at each demand <br />point if there is not sufficient water in the river to satisfy the <br />demand. For each demand, the model "allows" a certain amount of short- <br />age to occur before a shortage message is printed. This allowable <br />shortage is only valid with respect to the printing of shortage messages <br />and does not control the amount of water a demand receives during a <br />shortage condition. The allowable shortage is specified in the demand <br />data base for each demand. <br /> <br />There are two parts of the model that do impart some type of shortage. <br />In the determination of the 602(a) storage quantity, a certain amount <br />of shortage is assumed to occur over the entire Upper Basin (see <br />section 8.2.4). Second, the shortage strategy in the model directs <br />specific shortage amounts on certain Lower Basin users during shortage <br />conditions (see section 8.5). <br /> <br />Certain demands on tributaries can incur shortages in the model that <br />might not occur in actuality. This can happen to demands high in the <br />basin that receive their water from small tributary or offstream reser- <br />voirs that are not modeled by the CRSM. (Although the operation of <br />these small reservoirs is not specifically modeled, they are used in <br />the computation of CRSS natural flows.) In the CRSM, these demands get <br />thei r water di rectly from the ri ver. If the ri ver runs dry in months of <br />low flow, these demands will incur shortages that might not occur in <br />actuality if there was water available in the small supply reservoirs. <br /> <br />11.4 Bank Storage <br /> <br />The CRSM models the effect of reservoir bank storage by computing a <br />monthly change in bank storage. Change in bank storage is simply com- <br />puted as a specified percentage of the change in reservoir contents for <br />the month. The percentage used for each reservoir is entered into the <br />model from the CRSM control file. The percentages currently being used <br />have been determined in various studies of historical data and are given <br />in appendix B. <br /> <br />11.5 Evaporation <br /> <br />The CRSM models the effect of reservoir evaporation by using monthly <br />evaporation rates. These monthly rates are entered into the model from <br />the CRSM control file in units of feet. Reservoir evaporation is com- <br />puted each month by multiplying the reservoir's current surface area by <br />the evaporation rate for that month. Annual evaporation rates are given <br />in appendix B. <br /> <br />51 <br />