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<br />00lS76 <br /> <br />7. Hydrologic Analysis <br />7.1 Runoff Forecast Procedure <br /> <br />The CRSM develops a runoff forecast in order to simulate reservoir <br />operations at Lake Powell and Lake Mead. At Lake Powell, the runoff <br />forecast is used to avoid excessive releases at Glen Canyon Dam, and is <br />also used to predict the EOWY contents of Lakes Powell and Mead to <br />determine if equalization releases are necessary (see section 8.2). At <br />Lake Mead, the runoff forecast is used to determine any flood control <br />releases at Hoover Dam during the January to July period (see <br />section 8.6). <br /> <br />7.1.1 Lake Powell Forecast <br /> <br />CRSM simulates the runoff forecast for Lake Powell each month from <br />January through September. Each month, the runoff volume for the <br />current month being modeled through September is forecasted. Because <br />CRSM "reads" the natural i nfl ow that wi 11 occur duri ng the forecast <br />period, it al ready has a perfect forecast. To use the perfect fore- <br />cast in determining the reservoir operation would unrealistically <br />bias the results obtained from CRSM. So, a forecast error is com- <br />puted and added to or subtracted from the perfect forecast in order <br />to more realistically simulate the reservoir operation. <br /> <br />The Lake Powell runoff forecast can be broken down into the following <br />components: <br /> <br />(1) The natural flow into Lake Powell from the current month <br />being modeled through July, <br /> <br />(2) The long-term average natural flow into Lake Powell for the <br />months of August and September, which is added to the above, <br /> <br />(3) The estimated depletion in the Upper Basin during the forecast <br />period, which is subtracted from the above, and <br /> <br />(4) A forecast error which is added to or subtracted from the <br />above. <br /> <br />Items (1), (2), and (3) are all read from input data files by the <br />CRSM. Item (1) is read from the hydrology input data file. Item (2) <br />is read from the CRSM control file. Item (3) is read from the <br />demand data input file. Item (4) is computed each month, except <br />August and September in which no forecast error is assumed. <br /> <br />Forecast Error <br /> <br />The runoff forecast error is computed using equations derived from an <br />analysis of past Colorado River forecasts and runoff data for the <br />period 1947 to 1983. Analysis of these data reveals two strongly <br />established patterns: (1) high runoff years are underforecast, and <br /> <br />28 <br />