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PARSONS Memorandum To Larry Lang April 2, 2003 Page 2 Conclusions Subject to resolution of some of my minor-issue questions marked in the margins of my copy, and subject to practicable resolution of any significant remaining issues identified by Lou or Nolan, and subject to the implications of my discussion of "latitude" below, I am inclined to agree with AWA's recommendation in Section 12 that their site- specific PMP study values can be used by CWCB to detennine the PMF for the basin. The primary basis of my position is a standard that I believe is widely accepted by our profession that any prudently-conducted detailed study supersedes regional analysis and should be adopted by the review and approval agencies unless proven inaccurate. These are my words, but I believe similar wording could be found in various professional association or regulatory agency rules and regulations elsewhere. The whole concept and definition of a "site-specific" PMP corroborates my notion that any more-detailed analysis of the PMP for any area supercedes any less-detailed analysis. If this level of detail was perfonned on any other basin, say the South Platte for example, it is likely that the resulting depth of PMP rain would change. Had the PMP depth for Cherry Creek increased, I believe that CWCB would be justified, and obligated, in using the new value for PMF calculations. Under this rule, I believe that NWS should put on their "review agency" hat and either prove the study inaccurate or accept it under the "more detailed analysis" rule. I'm sure NWS may disagree that they are a review agency, and may never agree with the study, but I believe CWCB is allowed to apply the "more detailed analysis" rule on this issue, Resolution of the differences with NWS does not appear possible. I think that a discussion, either in a meeting or in the report, of what I call "latitude in the standards" would be beneficial. By this I mean is it possible that the procedures used by A W A are within "code," meaning that the defined procedures and standards in the HMR's have sufficient latitude to embrace the variations employed here? If it can be proven that the HMR's would absolutely forbid any of the significant aspects of this study, then CWCB would have a decision to make as to whether they would accept a non-standard approach. Regarding the second issue, orographic effects, I stated in my May 3rd memo that A W A completed a large number of detailed analyses regarding orographic effects, inflow wind directions, moisture depletion, downslope wind conditions, and topographic GIS analysis, all of which indicate that a decrease in rainfall due to downslope wind conditions might be justified. Yet they recommend use of a neutral condition. I definitely agree that all this analysis points out that the upslope enhancement factors used by the NWS study should continue to be challenged. Another important consideration in settling these issues, besides the "more detailed analysis" rule, is whether the methods used by A W A fit the universal definition of PMP, especially regarding the tenn "possible." One of my comments on Interim Report #2 was that A W A should provide supporting evidence that their findings on each issue fall