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In this model, the highest estimated runoff from snowmelt is 0.36 inches on March 4th, well below the 1.4", <br />10-year, 24-hour event used in our hydrologic models. <br />Using the 10-year average has a tendency to levelize the peaks in snowfall and temperature and therefore <br />the model loses some credibility. [n consideration of this weakness, a second model was developed using <br />that year that most closely represents the average year in snowfall The winter of 1986-87 with a snowta0 of <br />76.8" most closely approaches the 10-year average of 78.3". <br />The second model (Attachment 2.6) uses aaual snowfall, equivalent moisture and degreeday data for 1986- <br />87 and a value of K = 0.12 in the equation M = KD. In order to check the validity of the value for K, the <br />CMLRD inspection report for March 1987 was pulled to check for a documented observation of snow <br />depth. On March 19,1987 Mike Long and Jeff I)eckler reported "approximately 3~ inches of snow covered <br />the site" (see Attachment 2.7, March 1987 Inspection Report). This seems consistent with the model for <br />this same period. A knver value of K would add to the modeled snowpack while a highe[ value would <br />subtract The model indicates that a 3"~" snowpack would be probable in the form of new snow with old <br />winter snowpack having melted during the previous weeks. <br />In the second model, the highest estimated runoffs from snowmelt were 0.68 inches on Apn7 20 representing <br />the immediate melting of a 7' snow event and 0.72" on February 14. These values are also well below the <br />1.4", 10-year, 24-hour event used in our hydrologic models. <br />Predicting the runoff from snowmelt has not received the rigorous treatment in the literarure that rainfall <br />has. The "an" of prediction of snowmelt runoff is much less sophisticated and somewhat subjective when <br />site specific data and history is not available. The degreeday method neglects evaporation which can be as <br />high as 21" of snowpack per month (Clyde) and oversimplifies the effect of temperature (e.g. 12/9/90 shows <br />0 degree days although the temperature reached 44° on that day). <br />Based on the models developed using the methodology described, it appears that a snowmelt event <br />exceeding 1.4" in 24 hours is unlikely. Given Trapper's excellent compliance record and some extraordinary <br />reclamation measures to control erosion and runoff, we feel tha[ our current rainfall event assumptlons are <br />valid and should remain valid until our performance in the field shows otherwise. <br />