2002-06-17_GENERAL DOCUMENTS - M1979097

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Evaporative Losses The NOAA Technical Report NWS 33, Evaporation Atlas for the Contiguous 48 United States (U.S. Departrrmehtof Commerce) was used to determine gross evaporation: For this location; the . annual evaporation is 39.0 inches (Table No. 1). Monthly evaporation percentages as suggested by the Office of the State Engineer were used to calculate monthly evaporation. Precipitation.was estimated using Colorado Climate Center Website data. Arinual precipitation at this site was assumed to be equal to that at the nearest gage station. The site lies between the Fort- Collins WSFO and Windsor Station, and these values were averaged for the East Ridgen Pit.-The averagedvalue is 13.92 inches (Table No. 1). Effective precipitation was calculated as 70% of total precipitation: -Net evaporation was then computed as the difference between gross evaporation and effective precipitation and found to be 2.4 feet. The exposed surface acreage has been.reduced since the previous submittal to the SEO. The East Rigden site has a total exposed surface area of 90.6 acres, but has approximately 25.0 acres of pre-1981 exposed surface area (Figure No. 1). Mining is complete at East Rigden, and no additional groundwater has been exposed. This leaves 65.6 exposed acres that require augmentation. The total evaporative consumptive use for the site was calculated at 159.9 acte- feet per year (Table No. 1). Operational Losses The mining at East Rigden was completed in 1999. The material now processed at this site is conveyed across the Cache la Poudre River from the Resource Recovery site, M-99-078. The East Rigden Plan will make replacements for this material. The estimated annual production remains of 800,000 tons of material. The moisture content of the material removed from the site was assumed to be 4 percent by weight per the SEO's Guidelines. This results in an annual consumptive use of 23.5 acre-feet for water retained in product (Table No. 2). The total annual. consumptive use for dust control was estimated at 2.5 acre-feet (Table No. 2). The annual amount for dust control was distributed throughout the year based on the same percentages as the production of the aggregate mined. The East Rigden site will also consume an estimated 0.08 acre-feet at the septic system (Table No. 2). The total operational consumptive use for the East Rigden Pit was calculated to be 26.1 acre-feet per year (Table No. 2). The total annual stream depletions from the East Rigden site are 186.1 acre-feet (Table No. 2). MINING DEPLETIONS Due to the fact that the site is set back from the river, the mining depletions were assumed not to be instantaneous. A stream depletion model was completed for the East Rigden site to display Page 2