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Derr Pit - Groundwater Modeling Report <br />January 31, 2020 Page 5 of 24 <br />McGrane Water Engineering, LLC <br />1669 Apple Valley Rd. Lyons, CO 80540 Phone: (303) 917-1247 <br />E-Mail: dennis@mcgranewater.com Web: ttp://www.mcgranewaterengineering.com <br />value. A 10:1 Kh/Kv ratio is a common ratio used for alluvial aquifers. Therefore, to be <br />conservative, we decided to reduce the connection to the river by using a Ksb of 36 ft/day. <br />Based on average streambed widths measured from Google Earth (1/27/2017), we calculated a <br />COND value of 396000 ft^2/day as shown in the table below. <br />River Length <br />(ft) <br />Width <br />(ft) <br />Ksb/m <br />(day^-1) <br />COND <br />(ft^2/day) <br />Poudre 200 55 36 396000 <br />“Calibration” Results <br />Figure 8 compares the modeled predevelopment water level to the target water level contours <br />created from existing well data sets. In a perfectly “calibrated” model, the modeled and <br />observed contours would overlap. The 4620 ft elevation contours and the 4640 ft contour on the <br />western model boundary match very closely. There is some difference between the 4630 to 4660 <br />ft contours in the central to northern model area. We feel that to pull the modeled contours <br />further north would have involved increasing the model hydraulic conductivity (a measure of <br />permeability) above what we consider reasonable. Therefore, we believe our modeled water <br />table is likely more accurate than the predevelopment water table (white contour lines) target. <br />The figure below compares the measured DWR water levels with output from the model <br />(DerrSS3 run) at the cells containing wells. The plot shows that the model is very accurate <br />(close to the line representing a perfect match of slope = 1) between the 4610 and 4640 foot <br />elevation levels which includes the pits and most of the wells. <br />Additional time could have been spent trying to improve the match so all the data plotted closer <br />to the theoretically perfect line with slope of 1 shown. However, a perfect match is not possible