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simulations in Task 1.4 (Run Historic Monthly Yampa River Basin StateMod Model) and <br />Task 3.1 (Run Calculated Monthly Yampa River Basin StateMod Model). <br />5. Compare Baseflow Results using Variable Efficiency and Soil Moisture Accounting to <br />Previous Yampa River Basin Baseflow Results. Representative gaged and ungaged baseflow <br />locations were selected for a detailed comparison. Previous baseflow results were compared <br />to baseflow results using variable efficiency and soil moisture accounting. The following <br />section describes and graphically presents the comparison. <br />Results <br />Baseflows at stream gage locations are estimated by StateMod using the following formula: <br />where: <br />Qb=Qg+D-R-S+E-I <br />Qb =Base flow <br />Qg = Gaged flow <br />D =Upstream diversions <br />R =Upstream return flows <br />S =Upstream change in reservoir storage <br />E =Upstream reservoir evaporation <br />I =Imports <br />With the variable efficiency and soil moisture accounting algorithm active, consumptive use and, <br />therefore, return flows may change when compared to previous estimates that used 12 constant <br />efficiencies (one for each month) per structure and no soil moisture accounting. A simple <br />StateMod example data set was developed to hand-check the results of the baseflow calculation <br />using the variable efficiency and soil moisture algorithm. It was verified that StateMod is <br />operating in baseflow mode as outlined in the StateMod Documentation, (StateMod version <br />9.81, March 30, 2001)) Section 7.9 -Modified Direct Solution Algorithm. <br />The factors that effect how much baseflows vary from previous estimates include: <br />• The difference between a systems actual efficiency and the 12 constant monthly <br />efficiencies used in previous estimates <br />• The number and size of diversions between a gage and the next upstream gage <br />• The variability in available streamflow upstream of a gage <br />• The quantity of streamflow at a gage <br />The baseflow estimates generated using the variable efficiency and soil moisture accounting <br />algorithm were compared to previous baseflow estimates at each baseflow location. In general, <br />the most variance in the two simulations occurred during times when diversions were much <br />greater than irrigation water requirements, although as discussed in the Task 2 memorandum, <br />there does not appear to be a strong correlation with diversions and wet hydrologic years defined <br />during Phase IIIb. Overall, the correlations between the two baseflow simulations at each <br />L:ASecrion Folders\SECD\Carolyn\CDSS\VariableEff Taskl-3.doc 4 of 16 <br />