Laserfiche WebLink
<br />lHH104 <br /> <br />. <br /> <br />. <br /> <br />In order compare model results, a detailed review had to first be made of the structure <br />and assumptions of the HYDROSS model. This review was aimed at identifying differences in <br />underlying assumptions regarding hydrology, demand conditions and reservoir operations <br />which might contribute to differences between the Hydrosphere baseline numbers and those of <br />the Section 7 baseline. . <br /> <br />Several important differences were discovered during the review process. Perhaps the <br />most important difference related to the demand conditions represented in HYDROSS upstream <br />of DeerlodgePark. Although the HYDROSS model was configured to represent <br />approximately 35,000 af of depletion upstream of Deerlodge Park to account for demands by <br />the Hayden Station, Craig Station, and other water uses, these demands were not correctly <br />activated in the model to effect a depletion to the. Yampa River at Deerlodge Park. This <br />resulted in overestimating the Section 7 baseline flows at Deerlodge Park by 35,000 af per <br />year. <br /> <br />This problem in the HYDROSS model was not manifested in the modeled flows at <br />Maybell because of the structure of the model. However, several other important differences <br />in either operations assumptions or demand size representation were revealed during the review <br />process that directly affected the HYDROSS predicted flows at Maybell. As an example, <br />while the Hydrosphere model represents all depletions which have previously undergone <br />Section 7 consultation (or are "grandfathered in"), the HYDROSS model does not. <br />Furthermore, HYDROSS represents the 1982 demand condition in the basin while the <br />Hydrosphere baseline represents maximum demand conditions projected for 1989. Finally, the <br />HYDROSS model, and thus the Section 7 baseline, reflect a double counting of several <br />depletions associated with thermal power generation units at Hayden and Craig. <br /> <br />With these points in mind, it was determined that the most logical way to assess the <br />impacts of the enlarged Elkhead Reservoir Project on stream flows in the Yampa River was to <br />make a comparison using only the Hydrosphere model results of the baseline scenario and with <br />project scenario. <br /> <br />Pre-Project Conditions <br /> <br />The pre-project scenario, I.e. the "baseline", was defined in the Hydrosphere model by a <br />demand level representing estimated 1989 maximum.demand conditions. In addition, all basin <br />depletions which have previously undergone Section 7 consultation were represented as <br />demands. Storage contracts and operations of Elkhead Reservoir were represented in the <br />model based on current agreements or operational practices. <br /> <br />Elkhead Reservoir <br /> <br />I <br />Under the modeled baseline conditions, Elkhead Reservoir experiences very little <br />fluctuation in storage contents (Figure 5). This is due largely to the ability of natural flows to <br />satisfy most basin demands, both on the Yampa River and on Elkhead Creek. Some drawdown <br />of Elkhead Reservoir would be expected during hydrologic conditions similar to those <br />experienced during the mid-1930's. Elkhead Reservoir typicany spills during April or May of <br />each year. <br /> <br />Elkhead Creek <br /> <br />Table 3 shows modeled average monthly flows downstream of Elkhead Reservoir under <br />baseline conditions. Flows average less than 10 cfs during the fall and winter months and up <br /> <br />16 <br /> <br />. j~ ",,;jl <br />