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averaging about 900 per year, and then increased in the 1987-1991 time period to about 2,200 <br />per year. It was during the 1987 to 1991 time period that the number of measurements collected <br />by the USGS started to decrease, and the number of measurements collected by the DWR <br />increased. The DWR has been collecting approximately 70 water level measurements per year in <br />the South Platte Alluvial Region since 1988. The number of measurements in 2004 to 2006 shows <br />a substantial increase due to the SPDSS alluvial water data collection efforts under Task 35 and <br />Task 39. It should be noted that the spike in measurements from 2004 to 2006 represents a very <br />high measurement frequency on a limited number of wells. <br />Figure 3 represents a tabulation of the total number of measurements collected in a given <br />month. The majority of measurements are collected in either the spring or fall, prior to and <br />immediately following the irrigation season. The largest number of measurements have been <br />collected in the spring season. Water level measurements collected from the alluvial aquifer <br />before the irrigation season begins are important because they are least likely to be impacted by <br />localized irrigation pumping and thus reflect the long-term changes in the alluvial aquifer in <br />the South Platte Basin. <br />An important consideration is that for most of the data it is unknown whether a given water <br />level measurement was collected from or near a well that was pumping. HydroBase does not <br />have a field describing the pumping status of the measured wells. For the time periods selected <br />for detailed analysis and mapping for this TM, the pumping status was inferred through the <br />evaluation and screening of outliers, as discussed in Sections 1.3 and 1.4. <br />The analysis of water level measurement dates described above, combined with analysis of the <br />bedrock water level measurements from the Denver Basin Region, led the SPDSS team to <br />choose spring pre-irrigation time periods in 1968,1978, 1990, 2001, 2003, and 2005 as the years <br />upon which contoured water table maps would be developed. These time periods were selected <br />so that water level contour maps would exist for both SPDSS study area regions for the same <br />time periods. These time periods represent, collectively, the periods with the most data that <br />could be contoured and would correlate with data gathered by other SPDSS contractors. <br />General water level trends can be determined using hydrographs and potentiometric or water <br />table maps prepared for these years. <br />1.3 Database Queries <br />To develop water table surfaces for the selected time periods, several queries were made from <br />the SPDSS database. The first step was to identify all wells in the database that are located in the <br />SPDSS South Platte Alluvium Region. The August 2006 download from HydroBase includes <br />wells from all of Division 1. All the wells were imported into ArcGIS, and a spatial query was <br />used to select wells within the South Platte Alluvium Region. <br />In the SPDSS database, the WELLS and WELL_MEAS tables were linked using the well_id field <br />allowing water levels for the alluvial aquifer to be queried. Once this linkage was established <br />the first early spring (February to April) water level measurement for a given alluvial well <br />within the South Platte Alluvium Region was queried for each of the time periods (1968, 1978, <br />1990, 2001, 2003 and 2005). This set of queries resulted in the data sets used for gridding the <br />water table surfaces. Detailed instructions for querying the water level measurements for each <br />time period can be found in Appendix A. <br />SPDSS Phase 3 Task 44.3 TM -Final 7 <br />11/29/2006 <br />