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Selection of Temporal Data <br />The Denver Basin Region bedrock water level data set was evaluated to identify up to six time <br />periods that include a sufficient amount of data to map the potentiometric (water level) <br />surfaces. These time periods were selected to characterize groundwater levels through the study <br />period, and to be used for calibrating groundwater flow models. This was determined by <br />summing the number of measurements recorded for each aquifer on a monthly and yearly basis <br />over the period represented. Summaries of the results by year for the Denver Basin Region are <br />shown on Figure 3. Water level measurements began in 1929 and increased to about 80 per year <br />by the late 1950s. The number of measurements increased to about 300 per year by the late <br />1970s, and increased to 620 measurements in 1978. The number of measurements decreased <br />between 1979 and 1986, averaging about 300 per year, and then decreased again in the 1984 to <br />1986 time period to 90 measurements per year. It was during the 1987 to 1991 time period that <br />the number of measurements collected by the USGS started to decrease, and the number of <br />measurements collected by the DWR increased, according to the information collected. The <br />DWR has been collecting approximately 250 water level measurements per year in the Denver <br />Basin Region since 1988, equating to approximately 50 percent of the measurements since 1994. <br />As shown on Figure 4, a tabulation of the total number of measurements collected in a given <br />month, the majority of measurements were collected in either the spring or fall, prior to and <br />immediately following the irrigation season. The largest number of measurements was collected <br />in the spring (February -April) season. Water level measurements collected from the bedrock <br />aquifers in the spring pre-irrigation season are more likely to represent regional conditions since <br />they would not be impacted by higher-volume summer pumping. <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. Currently there is no <br />field in HydroBase to identify this. For the time periods selected for detailed analysis and <br />mapping for this TM, the pumping status was inferred through the evaluation and screening of <br />outliers, as discussed in Sections 1.3 and 1.4. <br />The analysis of water level measurement dates described above for the Denver Basin Region <br />combined with analysis of the South Platte Alluvium Region water level data led the SPDSS <br />team to choose the late winter/early spring time periods in 1968, 1978, 1990, 2001, 2003, and <br />2005 as the years for which contoured water table maps would be developed. These time <br />periods were selected so that water level contour maps would exist for both the Denver Basin <br />Region and South Platte Alluvium Region regions for the same time periods. These time periods <br />collectively represent the periods with the most data that could be contoured and would be <br />consistent with data gathered by other SPDSS contractors. General water level trends can be <br />determined using the potentiometric and water table maps prepared for these years. <br />1.3 Database Queries <br />To develop potentiometric and water table surfaces for the selected time periods, several <br />queries were made from the SPDSS database, HydroBase. The first step was to identify all wells <br />in HydroBase located in the SPDSS study area as shown in Figure 1. This selection allows a <br />temporary lookup table to be created that allows one to query only the wells in the WELLS table <br />located in the SPDSS study area. This lookup table was created and added to the SPDSS <br />database. <br />SPDSS Phase 3 Task 44.2 TM -Final 6 <br />11/28/2006 <br />