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3.1.4 Aquifer Storage <br />Water level data from HydroBase were used to evaluate alluvial aquifer storage. Water level <br />data from alluvial wells located throughout the active model domain were examined, using <br />information developed under the Task 39 and Task 44 evaluations. The hydrographs for a <br />subset of these wells is shown in Figure 3-7 to illustrate the long term changes in alluvial <br />groundwater levels. As concluded in the Task 44.3 TM and indicated ii1 Figure 3-7, alluvial <br />aquifer water levels show little change from year to year u1 most locations. Although some <br />wells show declines and others show increases in water levels, the majority of alluvial wells <br />show relatively stable levels compared to the seasonal changes. This indicates that, on an <br />annual basis, there is little change in alluvial aquifer storage and so almost any time period <br />could be used for t11e steady-state period based on this criterion. However, most of the <br />hydrograplls reviewed including many shown i11 Figure 3-5 and others included in the Task 39 <br />and Task 44.3 TMs show significant seasonal fluctuations. These water level changes are a focus <br />of the transient calibration efforts. <br />3.1.5 Well Development <br />Alluvial well development u1 the South Platte was evaluated using the aiulual total for decreed <br />hig11 capacity wells (those wit11 decreed pumping rates greater than 50 gpm). Periods of little <br />change in decreed capacity are preferred for the steady-state calibration period while periods of <br />large change i11 decreed capacity would be preferred for t11e transient period. <br />Using a water rights query from HydroBase, t11e decreed capacity and adjudication date of <br />wells in the South Platte alluvium within water districts that are included at least partially <br />within the study area were identified for each year (Figure 3-8). Well decreed capacity in the <br />South Platte started to increase i11 the mid-1930's and continued at a hig11 rate of annual change <br />through the 1960's. After 1970 t11e capacity of newly decreed High capacity wells decreased to <br />annual numbers comparable to before t11e mid-1930's except for isolated years in 1985,1990, <br />2002 and 2003 (Figure 3-8). This suggests t11at pumping stresses have been relatively uniform in <br />t11e alluvial aquifer since the early 1970's with the exception of the four years noted. <br />3.1.6 Conclusion <br />The data reviewed and discussed in the previous portions of Section 3.1 supports the selection <br />of average values from 1991 through 1994 for the steady-state calibration period, and monthly <br />values from 1999 through 2005 for the transient calibration period. <br />The following sections summarize the data that will be used as the steady-state and transient <br />calibration targets. <br />3.2 Groundwater Level Targets <br />There will be two categories of groundwater level targets based on the accuracy of their <br />measurement. Water levels in wells are measured as a depth to water below some measuring <br />point, which is the top of the well. The water level measurement is converted to a groundwater <br />level elevation by subtracting the depth to water from the measuring point elevation. <br />The first category of groundwater level targets is data from wells that Have had their measuring <br />po>11t elevations surveyed. Data from these wells have accurate measuring point elevations <br />P~ SPDSS T~8 2 Final TM 10-08-0t~.doc 9 <br />