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on additional information on irrigation systems, irrigated acreage, cropping and historical water <br />uses were later incorporated into the Phase lc Surface Water Model. <br />Comments and Concerns <br />The use of average monthly efficiency data provided by the Consumptive Use Contractor is <br />considered appropriate for the Surface Water Model because its objective is to be a planning <br />tool. However, the use of 12 monthly averages rather than monthly estimates does introduce <br />some error in the calculation of historic baseflows. This error is absorbed into the gain <br />calculation performed as part of baseflow generation and will be most pronounced in a dry year <br />when on farm efficiencies can be significantly greater than an average value. <br />Because the efficiency of surface water and ground water are different, the application of <br />StateMod involving calculated demands (rather than historic headgate diversions) is expected <br />to use demand information specified as an irrigation water requirement rather than as a <br />headgate or well head requirement. This data along with appropriate StateMod enhancement <br />will allow the diversion or well head demand to be calculated "on the fly" as a function of the <br />supply (surface or ground) that is diverting. <br />C:Acdss\Task8-2.doc Efficiencies of Water Use September 8, 2000 -Page 4 of 4 <br />