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the late season, when most of the supply is from the reservoir, the efficiencies are <br />generally at the 60 percent maximum. <br />54_ADY022 Slater Creek Aggregate Diversion. This aggregate diversion structure <br />represents structures whose "measured" diversions are calculated based on monthly <br />irrigation water requirements and basin-wide monthly efficiencies. <br />For both the Previous CRDSS and Enhanced analysis, the four explicit ditches are able to divert <br />the same as they did historically in every year of the simulation and overall basin calibration is <br />considered good. However, the overall results showed that the Previous CRDSS simulation <br />predicted average annual total basin diversions that were within 2 percent, while the Enhanced <br />run average annual total basin diversions are within 6 percent. Most of the shortages differences <br />occurred in Water District 54 on Slater Creek and Little Snake River, where more than 50 <br />percent of the demand is represented by aggregate structures. The calculated efficiencies for <br />most of the structures in Water District 54 are at the 60 percent maximum. As discussed above, <br />a more detailed study of conveyance and application efficiencies may result in a lower maximum <br />efficiency for this area. The preliminary analysis showed that if the basin-wide maximum <br />efficiency was set to 55 percent instead of 60 percent, shortages would be reduced. <br />A~~re~ate Diversion Comparison <br />As noted, more than 50 percent of the demand in Water District 54 is represented by aggregate <br />structures. The four aggregate structures in Water District 54 are shorted almost 30 percent more <br />in the Enhanced analysis than in the Previous CRDSS simulation. The diversion records for the <br />individual ditches that are grouped into aggregate structures are generally not as complete as <br />diversion records for other structures. Therefore, aggregate structure "estimated" diversions for <br />the Yampa River Basin were determined by dividing the monthly irrigation water requirements <br />estimated by StateCU by overall basin average monthly efficiencies. This tends to estimate an <br />inflated diversion during dry and average years because it assumes an average efficiency and full <br />supply. Conversely, the use of an average efficiency tends to estimate a smaller historic <br />diversion during wet years. The Previous CRDSS approach gives better calibration because any <br />error in "estimating" diversions and simulating with a constant efficiency tend to complement <br />each other, while in the Enhanced model the error in "estimating" and simulating diversions <br />work against each other. <br />The following example demonstrates this issues: <br />Dry Month Irrigation Water Requirement = 200 acre-feet <br />Average Basin-wide Monthly Efficiency = 0.35 <br />"Estimated" Diversion = 200 / 0.35 = 571 acre-feet <br />It is likely that during dry months, efficiency approaches maximum. Therefore, a more <br />appropriate actual dry-month "estimated" diversion assuming no shortage might be 354 acre-feet <br />(220 / 0.55). If 571 acre-feet can be diverted, the variable efficiency approach would potentially <br />show both an inflated quantity diverted and a reduced amount returned -both which would <br />reduce available diversions to downstream users in the current month and subsequent months. <br />Note that the average basin-wide monthly efficiency is generally skewed by wet years since dry <br />VariableEff Taskl-5 8 of 17 November 5, 2001 <br />