Water Action Plan and Three State Projects Draft

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To: Water Action Plan Committee Chairman 5/3/2000 From: Platte River EIS Team Subject: Modeling of draft Water Action Plan and Three State Projects The Platte River EIS team modeled the Draft Water Action Plan (Revision No. 3 dated April 18, 2000) after further guidance and clarification from Boyle Engineering. In many instances, the exact target yield or score of each separate project could not be "fixed" or held to the desired target due to interaction between the different projects. This effect is not deemed critical as the modeling demonstrates that the combined range of yield and/or score for the individual projects is available to the draft Water Action Plan. We are also not able to "score" each project individually in terms of its specific contribution to the total reduction in instream flow shortage. However, we do list either the yield of a project on site, the amount contributed to the Environmental Account (EA) if applicable, or an actual "score" for each project. Although the total combined score is approximately 144,000 acre-feet in average shortage reduction, we recommend that the Water Action Plan Committee consider this value as an over- estimate because we have not been able to address Environmental Account losses to the extent we believe is necessary to fully support that "score". At this time, we recommend that the Committee consider the "true" score to be in the range of approximately 135,000 to 137,000 acre- feet in order to account for additional losses not captured in the current models. However, it is our assessment that the proposed mix of projects, if implemented to the scale outlined in the draft plan, is followed then the target result in reregulating flows to reduce shortages will be achieved. We also did not consider any competition between the draft Water Action Plan and the ability of the State's Future Depletion Plans to also provide water at the scale envisioned over the first proposed increment (13-15 years). This is mentioned not as a perceived problem, only to clarify the analysis that was done. Following is a summary of how each proposed component of the plan was incorporated into the North Platte and Central Platte EIS OPstudy models. NORTH PLATTE RIVER EIS MODEL La Prele Reservoir (Average yield = 2,225 acre-feet per year at the reservoir) Because the study period used by Boyle Engineering to prepare the Draft Water Action Plan (DWAP) is less than the study period being used for the Programmatic EIS, it was necessary to independently model La Prele Reservoir. To do so, the following assumptions were made. 1. Inflows to La Prele Reservoir are based on a USGS gage that was maintained on La Prele Creek a short distance above La Prele Reservoir. The inflow is assumed to be 105.5°/a of the gaged flow. This assumption was adopted from the DWAP prepared by Boyle. Where USGS data does not exist (November-February 1972, October-February 1973-1992, and all of 1993 and 1994) averages are used.