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<br />Memorandum To: File <br />Page 8 <br />September 28, 2001 <br />instream flow right is for 130 of/day. In the calibration (historical conditions) models, the <br />modeled gage flow exceeds 1330 of/day in one small peak that precedes the major seasonal <br />rise in the hydrograph (see Figure 3). The peak is evident in Figure 9 as a single tiny spike <br />above 130 of/day during early May. The dry year 1977 (Figure 8) provides another example: <br />there is available water in the stream only in the first half of April and for a few days in June. <br />Early in April, the historical hydrograph (see Figure 2) is beginning to rise, but on April 15`h, <br />NewDemand begins taking whatever flow is available above the instream flow right. In early <br />June, NewDemand requires approximately 1600 of/day while the instream flow demand <br />requires 130 of/day. The modeled historical conditions hydrograph exceeds 1730 of/day for <br />only a few days beginning June 4`h <br />Finally, the USFWS method appears to significantly underestimate peaks. This is the result <br />of two effects. The USFWS method is based on a monthly model, so, like the Daily Average <br />model, it "sees" NewDemand as fully satisfied in May, 1983. Since NewDemand can divert <br />fully throughout the month, there is less water at the gage, on a monthly basis, than in the <br />Daily Pattern and Daily Input models, when NewDemand was shorted for much of the <br />month. As a result, the total area under the USFWS hydrograph for May 1983 is smaller than <br />the area under the Daily Pattern and Daily Input hydrographs. The second effect is related to <br />the USFWS disaggregation of monthly to daily flows. In this month, patterning the monthly <br />volume after the historical gage leads to higher than realistic flows (assuming the Daily <br />Pattern and Daily Input models represent realistic flows) during the first half of May. The <br />extra volume that's "pushed into" the first half of the month must be compensated for by <br />taking volume out of the latter half of the month, when the month's highest flows occur. The <br />phenomenon is also apparent in July 1983, as well as May and July of 1988. <br />Conclusions <br />The Daily Pattern and Daily Input models are clearly much better tools than the Average Daily model, <br />which offers little more information or detail than a monthly model. Calibration is much "tighter" for the <br />Daily Pattern and Daily Input models, and in this application, was excellent. The two models are very <br />similar in terms of results, but very different in terms of effort. The Daily Input model is complex to <br />assemble, and we found it extremely challenging to keep straight the relationships between monthly and <br />daily data. The relationships change depending on whether a diversion structure is explicit or <br />aggregated, and whether files are being input for baseflow generation or simulation. As a result, <br />assembling the Daily Input model probably requires on the order of two to four times the effort required <br />for the Daily Pattern model, even if daily data is fairly complete. If daily diversion data is missing, the <br />difference could be even greater. <br />For its good results and ease of assembly, Boyle recommends the Daily Pattern approach for a full basin <br />daily model of the Yampa River. One limitation of the approach which did not exert itself during the <br />pilot study, but will be a factor in the full basin model, is the dependence on daily gage information. The <br />pilot study simulation period was selected to coincide with the Clark gage period of record. <br />Furthermore, the Clark gage hydrograph probably resembles the baseflow hydrograph closely, because <br />consumptive use above the gage is minor relative to the size of the stream. With the full basin model and <br />TaskMem2Final.doc 1304+LE <br />