Tamarack Ranch Recharge Project Water Rights

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4.0 GROUNDWATER MODEL In 2003, a groundwater model was developed as an operating tool for the Project. CDOW, NWCD, and Colorado State University staff worked together to create the original groundwater model. Upon negotiations with objectors, CDOW and other water court applicants conducted aquifer testing to better define aquifer parameters. Based on the results of this aquifer testing, the original model was revised and updated. The revised groundwater model was used to determine the pumping depletions and pond accretions for the Project structures. 4.1 Original Tamarack Recharge Model CDOW, NCWCD, and Colorado State University, worked together to monitor site hydrology and develop a groundwater model for the Project. In addition, staff from the Colorado Division of Water Resources provided consultation. The Original Tamarack Recharge Model was developed as a project operation and management tool and was designed to be continually updated and improved as operations change and new data is collected. The primary objectives of the initial modeling effort were to: 1) Determine the timing, amount and location of return flows to the South Platte River, 2-) Optimize the - design - and - operation of the recharge project, and 3) Develop a tool to better define the impacts of the recharge operation on wildlife habitat on • the SWA, and physical structures on or near the SWA. Detailed discussions on the setup of the groundwater model are provided in the Tamarack Modeling Project Documentation (Halstead and Flory, 2003). Highlights of the Original Tamarack Recharge Model are: • The numerical groundwater model was developed using Visual MODFLOW 2.8.2 and expanded using 3.0 (Waterloo Hydrogeologic, Waterloo, Ontario). The model grid is oriented parallel to the South Platte River. The model is divided into two layers approximating the sand dunes and the river channel deposits. The elevations for the layers were imported from contour surfaces generated with Surfer (Golden Software, Golden, Colorado) using data from the geologic logs and USGS topographic data. • The northern (river), eastern, and western, boundaries of the model were simulated as constant head boundaries. The southern boundary was simulated as a no flow boundary with some vertical recharge. • The model grid covers an area 12.7 miles by 3.6 miles. The grid extends along the South Platte River from approximately 3 miles to the west of the Crook bridge and over 5 miles to the east of the Red Lion bridge. This grid covers the entire Project site and also includes the Condon and Overland Trail Ranch operations adjacent to the Tamarack property. These neighboring operations conduct extensive well pumping for irrigation and some groundwater recharge. • The time span of the original model is from February 1, 1997 to October 31, 2002. 9 IPage