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I I I I I I I I I I I I I I I I I I I Goldhill, all in vain. Due to lack of specific data, the NOAA Atlas II precipitation values were used as discussed above. 2. Appendix "B", Watershed Data Watersheds were delineated based on 40' contours from a 1"= 2,000' USGS quadrangle map, with additional clarification within City 1 i mi ts obta i ned from a 2' contour map at 1 "=100' prepared by Ray and Associates in 1975 and by field reconnaissance to determine effects of urbanization on flow patterns. Areas were determined by planimeter. (See Exhibit "C".) Time of concentrations and lag times were obtained by use of TR-55 and NEH-4. Experience by the author has indicated that the SCS runoff curve number. (CN) selection is the most critical of all hydrological parameters. For typical watersheds, percentage increases up and down individually applied to basin area, time of concentration, precipitation amount, and CN values indicate that the CN value is approximately twice as sensitive as precipitation changes, which in turn is approximately 2 and 3 times more sensitive than basin area and time of concentration, respectively. Accordingly, much effort was placed on CN selection. An SCS soil survey has not been prepared for Central City area, nor has the USFS prepared or contracted for a survey to be performed. However, detailed soil information has been obtained for the Dorothy Lee Placer area by Empire Laboratories in conjunction with a study for a proposed water supply reservoir. (See Reference No. 7 in the Bibliography, and Appendix "B" for excerpts of report.) The report indicates that topsoil is silty and underlaid with silty sand. Soil overburden above bedrock is generally ample to eliminate bedrock as a factor in CN selection. 5 The report is for site specific soils. However, observation of soils throughout the entire watershed at depths to 6 inches indicate little variance from the silty sandy soil found in the report. The soil textures fall into the hydro 1 ogi c soil group (HSG) of B or better. A hydrology report for Eureka Gulch watershed above the Dorothy lee Placer was also previously approved by the office of the State Engineer which classified the HSG as B. (See Reference No.8 in the Bibliography.) The HSG was assumed to be B in this study. The antecedent runoff condition (ARC), formerly called the antecedent moisture condition, was assumed to be average, or condition II. Five (5) surface conditions exist in the watersheds. These are forested, range, graded, impervious, and commercial areas. Forested area classification in TR-55 is based on foliage type, and should not be confused with "woods", which refers to orchards. (See NEH-4.) Central City area forests fit into the oak/aspen category of arid and semi-arid rangelands.