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which provide accurate groundwater elevation readings. These data will be included in the <br />calibration process through their water level elevations. <br />Groundwater level data from most wells measured withal the South Platte Piver alluvial <br />aquifer fall into the second category of targets. These wells have measuring point elevations in <br />HydroBase that are estimated from topographic maps. The use of topographic maps to estimate <br />elevations results in significant uncertainty in t11e elevation of t11e water surface in t11e well. This <br />uncertainty is equal to plus or nunus half of the contour interval for the map used for the <br />estimate. Typically t11e map contour interval was 20 feet resulting in an uncertainty of plus or <br />nunus 10 feet for most of t11e study area. However, the accuracy of t11e depth to groundwater <br />measurements is very high, typically less than 0.1 feet error. To take advantage of this <br />measurement accuracy and incorporate these data alto the calibration process, the change in <br />depth to water rather than the water level elevations will be used for wells that have not been <br />surveyed. Depth to water data from surveyed wells will also be added to this dataset to <br />provide a larger dataset of t11e same types of measurement data. T11is data set will reflect <br />seasonal and vear to vear variation in water levels that will be used >11 transient calibration. <br />Weighting of the water level data is discussed in Section 4.3. <br />The locations and munbers of wells for which groundwater level target data exist are shown in <br />Figures 3-1 through 3-3 and Table 3-1, respectively, using the 06-01-2008 version of HydroBase. <br />Model results will be compared against all measurements collected during the calibration and <br />validation periods. <br />3.3 Streamflow Targets <br />Recorded flows at all major stream gages within the model doma>11 were selected as targets due <br />to their good spatial distribution and relatively complete periods of record within the study <br />period. These gages were also used in the Task 46 stream ga>lz/loss study so there is a <br />consistent set of flow and gain/loss >lzformation for these gages and reaches. Simulated <br />streamflow will be compared to observed data from 9 gag>11g locations shown on Figure 3-9. <br />These gages represent flow conditions over the mainstem of t11e South Platte River and also at <br />two major tributaries within the active model domain. Details on t11e gage locations, periods of <br />record, filling of missing records and other characteristics of these stream gages can be found in <br />the Task 2 TM (LRE 2006). A summary of the streamflow values used as targets are provided >11 <br />Table 3-4 below. The transient calibration and validation periods have a monthly set of <br />individual flow values that will serve as the targets for these calibration periods. <br />P~ SPDSS T~8 2 Final TM 10-08-0t~.doc 10 <br />