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the original source contour lines. The contours from the grids agreed very well with the original <br />source contours, which indicates the grids are an accurate representation of the source data. <br />1.2.5 Grids Modifications with Ground Surface <br />As a final check of the bottom of alluvium and groundwater level grids, each was compared to <br />ground surface (USGS 30-meter NED). It was found both grids extend above ground surface in <br />isolated areas. Areas where the gridded bottom of alluvium extends above ground surface are <br />limited to the southern (upstream) extents of mapped alluvial tributaries in the Kiowa-Bijou <br />and to the Upper Big Sandy. These represent approximately one percent of the alluvial grid <br />area. <br />The groundwater water table grid is higher than ground surface in small localized areas across <br />the entire grid, immediately adjacent to the South Platte River from Greeley to Weldona, in the <br />southern Kiowa-Bijou tributary areas and in the Upper Big Sandy. The total area where the <br />water table elevation is greater than the ground surface elevation is approximately 5.6 percent <br />of the alluvial grid area. <br />In areas where either of the grids exceeded ground surface, the grid values in those areas were <br />set to ground surface. These areas of modification are shown in Figure 4. The areas modified <br />for ground surface in the bottom of alluvium grid are limited to the fringes of the South Platte <br />Alluvium and the adjustments made are considered appropriate based on available data and for <br />the purpose of regional scale modeling. The groundwater level grid will be used for the <br />purpose of helping to define the Task48 model domain, and therefore, the ground surface <br />adjustment is a sufficient correction for that purpose. If more localized study or modeling is <br />conducted, then additional data will be needed to perform more refined adjustments in these <br />areas. <br />1.2.6 Development of the Saturated Thickness Grid <br />After the bottom of the alluvium and the groundwater level elevation grids were computed, the <br />bottom of the alluvium grid was then subtracted from the groundwater level grid using Surfer <br />to calculate a saturated thickness grid for the alluvium. <br />Division 1 decreed wells and Division 8 (Designated Basins) wells with greater than 50 gallons <br />per minute pumping capacity and with aquifer codes of "QAL" or "Groundwater" were plotted <br />on this saturated thickness grid (see figure 5). In a few isolated areas, the saturated thickness <br />grid indicated the alluvium was either unsaturated or showed very little saturated thickness <br />where a cluster of wells existed. The contoured bottom of alluvium values obtained from <br />historic reports were modified to agree with the bottom of alluvium picks gathered from the <br />DWR well permit images in these areas. After these modifications were completed the bottom <br />of alluvium was re-gridded and the saturated thickness grid was recalculated. The areas where <br />modifications were performed are shown in Figure 6. <br />1.2.7 Comparison of Picks and the Bottom of Alluvium Grid <br />To verify how well the grid agrees with pick data, a residual was calculated as the difference <br />between the pick value and its corresponding grid cell value. Residuals were calculated for <br />individual picks shown in Figure 2. The results are summarized by source in Table 2, which <br />includes the number of picks, 10th percentile, 50th percentile (median) and 90th percentile <br />SPDSS Phase 3 Task 42.3 TM -Final 7 <br />11/30/2006 <br />