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Model results for premining conditions reflect that no-mining related activities have <br />• occurred within the watershed boundary, and that associated input parameters represent <br /> existing conditions prior to any mining activity. Assumptions for postmining conditions <br /> that are reflected by the postmining input parameters include: 1) All disturbed areas <br /> have been regraded, topsoiled and reseeded according to criteria outlined in this revised <br /> permit ap plication paekage; and 2) All temporary structures (007 pond, culverts, etc.), <br /> roads and other facilities have been removed and reclaimed. <br />For both the premining and postmining model runs, the drainage area above the 007 pond was <br />delineated as one watershed. Due to the relatively small size of the drainage area above <br />the pond, only one branch was established in the watersheds for both pre- and postmining <br />estimates. Two junctions were established at the outlet of the watersheds (location of <br />the 007 pond), the first to serve as the required junction on the one branch, the second <br />for providing a comprehensive output of the results. <br />Watershed areas and topographic measurements used to determine the input parameters for <br />both runs were obtained from 1:400 scale maps of the Nucla East mining area (see Exhibits <br />17-1 and 17-2). The postmining topography map, Exhibit 20-10 was used for calculating <br />• pos training input parameters that reflect basin characteristics (average watershed slope, <br />time of concentration, etc.). A storm depth of 1.40 inches (5-year, 24-hour storm) With a <br />Type-II distribution was used for both pre- and postmining conditions. This storm value <br />was obtained from the NOAH Precipitation Frequency Atlas of the Western States (NOAA, <br />1973). <br />Hydrology input parameters include watershed area, curve number and time of concentration. <br />Curve numbers for premining conditions were selected by assigning curve numbers to soils <br />and corresponding vegetation types mapped in the Nucla East mining area (see Exhibit 9-3, <br />Tab 9, Soil Resources Survey, and Exhibit 10-2, Tab 10, Vegetation Baseline Survey). For <br />postmining conditions, a curve number was estimated for two vegetation and soil types that <br />will be reestablished in the postmining watershed above the 007 pond, irrigated pasture <br />(loam to sandy loam) and rangeland (loam to sandy loam). Curve numbers were assigned to <br />each type according to criteria for selecting curve numbers outlined by the U.S. Soil <br />Conservation Service and presented in design reports for each existing and proposed <br />sediment pond (see Attachments A through C, Tab 13, Facilities). Table 17-27 presents the <br />• soil and vegetation types and associated curve number assignments used to determine each <br />Weighted average curve number for the 007 watershed. The percent areal distribution of <br />17-83 Revised 04/11lBB <br />