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In the SPDSS database, the WELLS and WELL_MEAS tables were linked using the well_id field <br />allowing water levels for a particular aquifer to be queried. Once this linkage was established, <br />the first late winter/early spring (February to April) water level measurement for a given well <br />was queried for each of the time periods (1968, 1978,1990, 2001, 2003, and 2005. This set of <br />queries resulted in the data sets used as a basis for gridding and contouring the water table <br />surfaces. Detailed instructions for identifying the water level measurements for each time <br />period can be found in Appendix A. <br />Prior to contour grid development, all measurements were screened for outliers. Wells with <br />water level elevations greater than 8,000 feet or less than 3,000 feet were identified and excluded <br />from contouring, as these were determined to be data entry errors. These data are listed in <br />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. The software package Surfer® (Golden Software Inc; Version 8) was used for <br />this process but any software package with similar capabilities could be used. 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 their abilities to be consistent with the <br />individual data and to reduce the effect of localized scatter in the data. <br />Contours of the undivided Arapahoe and Dawson include water level measurements from the <br />Lower Dawson and Lower Arapahoe Aquifers, respectively. Grouping the undivided and lower <br />aquifers allows a continuous potentiometric surface to be created for deeper portions of these <br />aquifers. Aquifers with 10 or fewer water level measurements for a given time period were not <br />contoured, as the data are too sparse to produce reliable contours. A linear variogram was then <br />developed for each aquifer at each time period with more than 30 water level measurements. <br />This was accomplished by defining the slope of the variogram to provide the best fit based on <br />the spatial distribution for each data set. Aquifers with less than 30 measurements used a linear <br />variogram with unit slope. The contouring parameters used in Surfer® to create the water level <br />contours, together with the time periods and aquifers with sufficient data to contour, are shown <br />in Table 2 below. <br />SPDSS Phase 3 Task 44.2 TM -Final 7 <br />11/28/2006 <br />