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Control points representing the South Platte River bottom elevation were added to the water <br />level data set before gridding. South Platte River bottom elevation data, provided by the CWCB <br />Flood Control Group, were obtained every five miles along the river. These data help control <br />the water level contours along the mainstem of the South Platte River in time periods where <br />data are absent. All control points are noted in Appendix B. <br />1.4 Grid Development and Contouring <br />The water level measurements for the selected years were gridded and contoured using the <br />kriging method in the software package Surfer® (Golden Software Inc; Version 8). The kriging <br />method provides a smoothed representation of the water table surface. This method was chosen <br />after an evaluation of several methods based on each method's ability to be consistent with the <br />individual data and with regional trends in the data. <br />As part of the kriging process, a linear variogram was developed for each time period using the <br />characteristics unique to that data set. The alluvial aquifer contours were generated using a <br />circular search radius of 20,000 meters, using a maximum of 64 measurements to estimate a <br />water level value at each grid node. This relatively small search radius limits the influence of a <br />water level measurement in one tributary alluvial aquifer on the grid levels in neighboring <br />tributary alluvial aquifers. This relatively small search radius is particularly important to <br />accurately map the alluvium that overlies the Denver Basin. The contouring parameters used in <br />Surfer® to create the water level contours are shown in Table 2 below. In addition, Table 2 <br />summarizes the number of water level measurements available in the spring (February to April) <br />time period for the years selected for contouring including fifty four South Platte River bottom <br />elevation control points were used, as discussed in Section 1.3. The resulting water level contour <br />maps and associated discussion are presented in Section 3.1. <br />After the initial gridding and contouring was performed, each grid was reviewed for <br />measurements causing bull's-eye contours, or what appeared to be anomalous bends in the <br />contours. Water levels that caused the bull's-eye or anomalous contour shapes were evaluated <br />by comparing the water level at the given time period against the water level at that well at <br />other times. Anomalous water level measurements for the period being contoured were <br />excluded and are identified in Appendix C. If the water levels measurements identified for <br />contouring were consistent with water level data from other time periods for that well, even if <br />the water level differed from the levels in neighboring wells, it was maintained in the data set <br />for the gridding. It was assumed such measurements represented wells near areas of persistent <br />pumping, and removing such measurements would give a false representation of actual water <br />levels. <br />Another step to check for outliers was to compare water level elevations against ground surface <br />elevations, using a USGS 30-meter DEM of the land surface. All measurements with a water <br />level more than 10 feet above the DEM ground surface were excluded. The 10 foot value was <br />selected based on the range of error of the DEM ground surface contours. All excluded water <br />level elevations are noted in Appendix C. <br />As a final check of the water level grids, each was compared to ground surface (USGS 30-meter <br />NED). This check identified several small areas along mainstem of the South Platte River and <br />SPDSS Phase 3 Task 44.3 TM -Final <br />11/29/2006 <br />