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The following process was used to select both t11e steady-state and transient calibration time <br />periods. A summary of the data are presented later in this section: <br />1. Daily data for representative precipitation and streamflow gages were downloaded <br />from HydroBase. The average annual streamflow and precipitation for each year of the <br />study period were evaluated at monitoring locations distributed throughout the study <br />area. <br />2. To help select t11e steady-state calibration period, water levels in alluvial wells with <br />long-term records were evaluated to identify periods with relatively minor water level <br />change. By definition there is no change in aquifer storage in a steady-state period so <br />water levels should show no long-term increases or decreases. <br />3. The relative level of pump>11g in the South Platte alluvial aquifer was estimated by <br />querying the water rights database in HydroBase and summarizing the decreed <br />pumping rates of high capacity wells (greater than 50 gpm) for each year. <br />4. T11e total number of wells with water level measurements within fl1e study area was <br />summed for each year. This identified years with more data, which could better support <br />fl1e calibration process. Years were selected for further evaluation if they had at least 300 <br />wells with at least one measurement. <br />5. The number of wells with four or more groundwater elevation measurements for each <br />of fl1e years selected in the previous step was determined. Four measurements per year <br />were used as a threshold to assist >11 evaluating seasonal groundwater level changes in <br />the transient calibration time periods. <br />T11e spatial variability of groundwater elevation data for eac11 year was assessed using <br />plots of well locations and fl1e number of water level data points at each of fl1e <br />observation wells. A goal was to select time periods with a uniform spatial distribution <br />of wells that include multiple measurements in a year. <br />The process used to select the steady-state and transient calibration periods represented a <br />balance of the individual datasets described above, using engineer>11g judgment to make the <br />final selection of periods. These periods were reviewed and agreed to by the State prior to <br />finalizing the selections. <br />For the steady-state calibration period, the five-year period from 1991 to 1994 was selected as <br />representative of relatively consistent conditions for flows, precipitation, number of new wells, <br />and water levels within the alluvial system. For t11e transient calibration period, the seven-year <br />period from 1999 to 2005 was selected. This period includes a wide range in flows and climatic <br />conditions, including the record drought in 2002, and significant changes in the number of <br />wells. For the model verification period, the entire time period (1950 - 2006) will be simulated. <br />The follow>lzg describes the available data and t11e hydrologic characteristics of the selected <br />calibration time periods. <br />3.1.1 Water levels <br />Groundwater level data is one of the key datasets upon which model calibration will be based. <br />The ideal time period for model calibration is one for which a set of measurements exists that <br />cover t11e model area uniformly and over all time periods that are simulated. During the <br />selected steady-state and transient time periods there is a reasonably good distribution of water <br />level data, both spatially and temporally. Figures 3-1 through 3-3 summarize the spatial <br />P~ SPDSS T~8 2 Final TM 10-08-0t~.doc 6 <br />