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<br /> <br />Water surface elevation <br />(feet) <br /> <br />Release <br />(ft3/s) <br /> <br /> <br />The runoff forecast error is computed using relationships derived <br />from an analysis of past Colorado River forecasts and runoff data <br />for the period 1947 to 1983. The data indicate that high runoff <br />years are usually underforecast, and low runoff years are <br />overforecast. In this investigation, the system was operated with a <br />1 in 20 forecast error, which is defined as a 1 in 20 chance of <br />error or an error of 5 percent. The January forecast error is <br />5,000,000 acre-feet for Hoover Dam, which is reduced as the season <br />progresses and additional snow depth data are collected. By July, <br />the forecast error has decreased to 750,000 acre-feet. The forecast <br />error is used as an adjustment to forecasted inflows to determine <br />reservoir release rates. <br /> <br />If available flood control storage space diminishes at any time of <br />the year to less than 1,500,000 acre-feet, the minimum flood control <br />releases from Lake Mead are determined daily from table 6.1. These <br />releases are determined using available flood control storage space <br />in Lake Mead and inflow to Lake Mead. Some changes to this <br />operating rule are permissible based on the current reservoir <br />release rate, forecasted inflow, and available flood control storage <br />space, as specified in the flood control working agreement. <br /> <br />The upper basin, above Glen Canyon Dam, is operated to avoid spills. <br />It has creditable flood control space (joint use flood control and <br />conservation storage) of about 3.2 million acre-feet in Blue Mesa, <br />Flaming Gorge, Fontenelle, and Navajo Reservoirs. Changes in system <br />operation have historically been made semimonthly in April, May, and <br />June, but are made more frequently if needed. <br /> <br />Table 6.1.--Minimum flood control releases from Hoover Dam <br /> <br />1219.61 - 1221.40 Equal to inflow up to 28,000 <br />1221.40 - 1226.90 Equal to inflow up to 40,000 <br />1226.90 1229.00 Equal to inflow up to 65,000 <br />Above 1229.00 Maintain outflow equal to inflow <br /> <br />Notes: <br /> <br />Elevation <br />Elevation <br />Elevation <br />Elevation <br />Elevation <br />Elevation <br /> <br />1205.30 <br />1219.61 <br />1221.40 <br />1226.90 <br />1229.00 <br />1232.00 <br /> <br />- Spillway crest elevation <br />- Minimum required flood control pool <br />Top of spillway gates in raised position <br />- Spillway discharge = 40,000 ft3/s <br />- Top of flood control pool <br />- Top of dam <br /> <br />71 <br />