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C\l C"-. ~ ~ ~i:r: 21. Forecasts. --All 1n.t"low forecasts for lake Melld, as measured at the Grand Canyon, Ariz., gaging station (pl. 2) are prepared b;y the United States Bureau of Reclamation, BouJ.der Cit;y, Benda. The methods used lire based upon intensive studies involving statistical correlatiOJ:\s. Forecasts are prepered the first da;y of each IIIOnth, or as otherwise needed, during the period 1 January through 1 August. In all. forecasts derived from past flows, the undepleted flow at Grand Canyon, Ariz. is predicted for the period Apr1l through July. Records show that this flow is practically the same as the inflow to Ialte Powell. The final forecast is obtained by subtracting the estimated depletion during the Apr1l..July period by translllOuntain diversions and effective storage spece in upstream reservoirs, eXCluding Ialte Powell. Effective storage spece in upstream reservoirs is computed for Flaming Gorge, Blue Meaa, and NlIvajo reservoirs, which are Colorado River Storage Project reser- voirs. This spece is defined as the lesser of the actual spece available, or the usable spece available. The usable space is the difference betveen the min1mum forecasted inflow vol\lllle for any specified runoff period and expected normJ. releases. The min1mum forecast 18 def1.ned as the esti- mated inflow volume that, on the average, will be exceeded 19 times out of 20. 28. The United States Bureau of Reclamation is continually improving and revising forecasting methods. Each season, the mat reliable fore- cast Dlllthod is chosen in advance of the forecast date by the Bureau of Recl.amation. Forecasts are generall;y based upon accuaulated precipitation, water equivalent of snow, antecedent runoff, or a combination of these elements. A list of 13 precipitation stations used in developing 1 Jan- uary, 1 February, and 1 March runoff forecasts is siven in table 6. Locations of these precipitation stations are shown on plate 2. Infor- mation on snow courses used in forecasting is given in the U.S. S011 Conservation Service publication "Water Supply Outlook." Table 1 lists the average contribution of subarea drainages to April-July runoff at Grand Canyon, Ariz. Subarea drainages are shown on plate 2. 29. The maximum forecast, which is used in determining flood- control releases, is selected by the Bureau of Reclamation. This forecast is defined as the estimated inflow volume in acre-t~et that, OJ:\ theav- erage, will not be exceeded 19 times out of 20. '!'he IIIBJdmwn forecast is calcul/lted by adding to the mean forecast the so-called l-in-lO error, as determined by the Bureau of Reclamation, using statistical methods. '!'he l-in-10 errors for forecasts made in 1968 are given in the follOWing table: Dllte Forecast method l-in-lO error 1 January....: Accumulated precipitation.............: : Million acre-feet 4.0 3.4 3.2 1. FebrtlBry_..:... .do.................................: ~ }4e.rcb......:... .do................. ~...............: 11