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Stream Channel Parameters and Changes <br />Exhibit 55B Due to Longwall Mining-Induced Subsidence Page 7 <br /> <br /> <br /> <br />831-032.923 Wright Water Engineers, Inc. <br />December 2021 <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