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Stream Channel Parameters <br />Exhibit 55 and Changes Due to Mining-Induced Subsidence Page 3 <br />• in Table 2. While there are differences in soils and vegetation within the study areas, for the <br />purpose of this conceptual analysis, conservative estimates of water yield and sediment yield <br />have been adopted which are reasonable for both azeas. <br />Average Annual Runoff <br />The mean annual runoff expressed in acre-feet per year per square mile is a fundamental <br />parameter for determining annual average sediment yield. Total annual precipitation varies <br />widely throughout the 526-square-mile basin of the North Fork at Somerset. In the higher <br />elevations of the entire North Fork basin, precipitation can total up to ~0 inches per year. Annual <br />precipitation for the drainage basins near the West Elk Mine typically fall in the 22-inch to 30- <br />inch range. Precipitation in excess of that portion lost to evapotranspiration and deep <br />percolation, and that ultimately becomes streamflow, also varies widely in the basin. <br />Woodward-Clyde conducted a water balance analysis for Horse Creek and Lick Creek for 1978 <br />• to 1980. The study, based on the Lick Creek stream gaging station, indicated an average annual <br />runoff of 8.9 inches, representing 47~ acre-feet per square mile. <br />MCC's Exhibit 18 indicates an annual runoff approaching 1,000 acre-feet for one square mile for <br />a variety of North Fork watersheds. However, that finding is modified by their reference to <br />much lowered water yields in South Prong and Horse Creek for the 1977 to 1978 period, which <br />were lower than the driest year yields of the regional basins. <br />The North Fork gage near Somerset provides the best long-term runoff data in the region, with <br />~8 years of continuous gaging records. The 58-year period of record identities a mean annual <br />runoff of 625 acre-feet per square mile; the runoff magnitude is significantly affected by the <br />higher precipitation values at higher elevations in the drainage basin. <br />Analyses of water yield by William Lorah of W WE for the Division No. 4 Water Court approved <br />water augmentation plan, indicated that typical annual water yields for tributaries of Dry Fork <br />• were approximately 200 acre-feet per square mile per year. This value compares favorably with <br />