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HYDROLOGI' <br />Rainfall <br />Rainfall information for the State of Colorado is published <br />by the National Weather Service in a report titled NOAA Atlas <br />2, Precipitation-Frequency Atlas of the Western United <br />States, Volume 3 - Colorado. The publication contains maps <br />giving total rainfall depths for the 2- through 100-year <br />frequencies and the 6-hour and 24-hour durations. For the <br />Bourg Coal property, the 24-hour duration amounts are as <br />follows: <br />10-year rainfall = 1.7 inches <br />25-year rainfall = 2.0 inches <br />100-year rainfall = 2.4 inches <br /> <br />Runoff Volumes <br />Runoff volumes were computed using equation 10.10 and the <br />procedures outlined in Chapter 10 of the Soil Conserva tion <br />Service's (SCS) rational Engineering Handbook, Section 4 - <br />Hvdrologv, dated 1971. Antecedent Moisture Condition - II <br />(A}1C) curv e numbers (CN) were estimated for the tribu tary <br />areas of e ach pond considering the worst case for land use, <br />and ground cover over the life of the proposed mine. The <br />tributary areas include both on- and off-site drainage ar eas. <br />Ah1C-II is considered applicable for average conditions and <br />allows for a total rainfall of 1.4 to 2.1 inches within the <br />preceedi.ng five days of the storm. <br />Based on information from the SCS .lackson Coun[v Soils 51ap, <br />A91C-II curve numbers of 72 and 91 were selected Em- <br />