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,, i • <br />Task 48: Calibrate Alluvial Groundwater Model <br />Objective: <br />The objective of this task is to calibrate the pre-development alluvial groundwater model prepared <br />in Phase 3 for the study period of 1950 to 2002. The alluvial groundwater model is intended to <br />enhance understanding of groundwater flow in the aquifer, identify and prioritize data gaps, and <br />to help evaluate various water resources activities including, but not limited to the following: <br />^ Effects of pumping on the location, magnitude and timing of stream depletions. <br />^ Effects of recharge activities on the Location, magnitude and timing of stream accretions. <br />^ Effects of water transfers on groundwater levels and stream depletions/accretions. <br />^ Effects of irrigation efficiency on the location, magnitude and timing of stream accretions. <br />^ Interaction between the bedrock and alluvial aquifer. <br />Discussion: <br />In Phase 3, apre-development groundwater model for the alluvial groundwater system from <br />Chatfield Reservoir to the state line was developed. The model domain includes all major <br />tributaries that overlie the Denver Basin Region including Plum Creek, Cherry Creek, Sand <br />Creek, Box Elder Creek, Beebe Draw, Kiowa Creek, Bijou Creek, and Lost Creek, along with the <br />Cache la Poudre River located upstream of Greeley. Alluvial tributaries not modeled include <br />those that drain from the western foothills including Bear Creek, Clear Creek, St. Vrain Creek; <br />Little Thompson Creek and Big 'Thompson Creek. These tributaries are incorporated into the <br />model as flow boundary conditions. In addition, the alluvial aquifer systems located in Water <br />Division 2 in the Upper Big Sandy and Upper Black Squirrel Designated Basins are not included <br />in the SPDSS alluvial groundwater model, since the groundwater flow in these areas drains to the <br />Arkansas River. The Phase 4 model will build on the Phase 3 model by incorporating conditions <br />for the 1950 to 2002 period and calibrating this model to observations during this period. <br />Estimates of stream flow, diversion, recharge, agricultural pumping and consumptive use <br />information for the study period are being developed by other SPDSS contractors during Phase 3, <br />and will be available for incorporation in this Phase 4 alluvial modeling effort. The data being <br />developed by other contractors will be reviewed and incorporated into the current modeling <br />efforts. The USGS is also developing a model of the Denver Basin aquifers that will provide <br />discharge quantities between the bedrock aquifers to the overlying alluvial aquifers being <br />modeled. Results from the USGS effort are expected to be available for use in Phase 4. <br />This Task also anticipates appropriate State personnel will participate in bi-weekly to monthly <br />working meetings requested by the contractor during all phases of the model development so <br />deliverables represent a consensus between Contractor and the State. The activities conducted <br />under this Task will conform with ASTM D 5447-04 Standard Guide for Application of a <br />Groundwater Flow Model to a Site Specific Problem and the modeling protocol outlined in <br />Section 1.4 of Applied Groundwater Modeling (Anderson and Woessner, 1992). <br />SPDSS_Phase4_gw_SO W_09-15-06.doc <br />