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Stream Channel Parameters and Changes <br /> Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 7 <br /> BASIN HYDROLOGIC CHARACTERISTICS <br /> WWE evaluated and studied each of the 8 basins and corresponding stream channels to define <br /> basic hydrologic characteristics related to the streams. This information is summarized in Table 2. <br /> While there are differences in soils and vegetation within the study areas, for the purpose of this <br /> conceptual analysis, conservative estimates of water yield and sediment yield have been adopted. <br /> Average Annual Runoff <br /> The mean annual runoff expressed in acre-feet (AF) per year per square mile is a fundamental <br /> parameter for determining annual average sediment yield. Total annual precipitation varies widely <br /> throughout the 526-square-mile basin of the North Fork Gunnison River(North Fork)at Somerset. <br /> In the higher elevations of the entire North Fork basin, precipitation can total up to 50 inches per <br /> year. Annual precipitation for the drainage basins near West Elk Mine typically ranges from 22 <br /> to 30 inches. Precipitation in excess of that portion lost to evapotranspiration and deep percolation <br /> (i.e., precipitation 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 to <br /> 1980. The study, based on the Lick Creek stream gaging station, indicated an average annual <br /> runoff of 8.9 inches, representing 475 AF per square mile. <br /> MCC's Exhibit 18 indicates an annual runoff approaching 1,000 AF for one square mile for a <br /> variety of North Fork watersheds. However, that finding is modified by their reference to much <br /> lowered water yields in South Prong and Horse Creek for the 1977 to 1978 period, which were <br /> 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 69 <br /> years of continuous gaging records. The 69-year period of record identifies a mean annual runoff <br /> of 630 AF per square mile; the runoff magnitude is significantly affected by the higher <br /> precipitation values at higher elevations in the drainage basin. <br /> Analyses of water yield by WWE for the Division No. 4 Water Court approved water augmentation <br /> 831-032.921 Wright Water Engineers, Inc. <br /> December 2020 <br />