Final Environmental Impact Statement Volume II Appendix K, Part 2

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COMMENTS OF U.S. ENVIRONMENTAL PROTECTION AGENCY EPA -38 Itwice the other variable). For the purposes of the calibration a (cont.) i slope near one and an intercept near zero is desirable. EPA -39 2. In examining the calibration curve (Figure A -6), the model appears to underpredict the reservoir contents for the period 1980 to 1985. The final EIS should provide an appropriate discussion, indicating the reason for this underprediction and commenting on its significance. EPA-40 3. To assess the magnitude of potential errors in the simulated vs measured lake levels, the maximum difference for 1988 was extracted from Figure A -6 to be approximately 100 KAF for the month. This converts to over 1600 cfs. This appears to overestimate water in storage, which means less water was actually available for release than was anticipated. Can the model calibration be refined to yield better results? The analysis should include standard errors. EPA-41 4. Calibration has not been presented for the various segments of the system for which discharge data were available. These should be provided where data are available, and especially in x the reach between Overton and Grand Island where critical decisions are based on model results. v� 0' EPA-42 5. Page A -5. The relationship of historic and daily flows at the Overton gage were apparently used to calculate daily flows from simulated monthly flows for all alternatives. Considering the potential sources of errors in the model and the fact that such a relationship is likely to be different for each alternative, the method may not be appropriate. EPA-43 6. Evaporation is an important sink of water especially in the reservoirs. The final EIS should include a section on evaporation. Assuming that evaporation pan data are available, the DEIS should provide the pan coefficient used to approximate lake evaporation. Economic Analysis EPA44 1. The economic analysis (Section 2.13) and supporting appendices E and G lack clarity and description. As a result, it is difficult to determine what the economic and monetary values presented in the RDEIS represent and whether they are accurate. For example, Appendix G provides a set of computed values with no description of the assumptions, components, or the method of computation. In addition, there is no discussion of the data inputs used to generate the Appendix G tables. Without including such information, it is difficult to make an independent determination regarding the accuracy or relevance of the information presented. 22 RESPONSES TO U.S. ENVIRONMENTAL PROTECTION AGENCY EPA -39 Allowable reservoir levels were higher in the early 1980's. Subsequent calibration in the SDEIS reflected higher allowable levels in the early 1980's followed by lower values in the later 1980's and in the 1990's. EPA -40 Standard errors were included in the SDEIS. The model remains an imperfect tool, not capable of capturing all decisions made by the Districts. EPA41 Flows at Overton were added to the calibration analysis in the SDEIS. EPA -42 Additional work was performed by the staff to refine the daily flow estimates. Staff recognize that these represent only estimates. EPA43 Monthly evaporation losses were included in the original OPSTUDY model (developed by the Bureau of Reclamation). Staff made no new adjustments to the evaporation data. The methods used to develop input values were documented by Central (Central, 1988). Evaporation losses averaged 63.1 KAF in the Baseline. EPA -44 The economic analysis and cost estimates have been revised, and associated tables reformatted and updated.