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C~ ~ _. <br />Approach: <br />Task 48.1: Model Development <br />~ Model layering and gridding, the initial aquifer propertiies, boundary conditions and pre- <br />development fluxes will be developed and used to configure apre=development MODFLOW <br />model of the aIluvial aquifer system. Model inputs for aquifer configuration, aquifer <br />properties and water levels will be developed using adata-centered approach as refined under <br />Task 50, based on data contained in HydroBase and as presented in SPDSS Phase 1 and Phase <br />2 Technical Memoranda. It is anticipated that most features will be maintained as GIS <br />coverages. <br />• A conceptual description of the groundwater system (in a form that is independent of the <br />model and computational grid) will be developed as a first step. This will include a <br />description of the extent and hydraulic properties of the alluvial aquifer, a water balance of the <br />key pre-development inflows and outflows, and a discussion of the interaction with surface <br />water features. It is anticipated that the conceptual model will be presented in a draft <br />Technical Memorandum (of up to 10 pages in length) to the State and their designated model <br />stakeholder group for review and discussionprior to developing inputs for the numerical <br />model. The conceptual model description will be revised based on State comments and will <br />become section 3.6 of the Model Report, described under Task 48.2. <br />• The location of streams: and canals, along with channel and bed seepage properties will be <br />linked to the conceptual model. Hydraulic properties for aquifer materials will be developed <br />either as property zones, or as continuous distributions using kriging, based on discussions. <br />with the State. Initial water levels will be developed based on the Task 44 (Aquifer Water <br />Level Data Analysis) work products. <br />• Pre-development recharge and evapotranspiration information will be provided by the <br />Consumptive Use/Water Balance contractor. <br />• If available, bedrock discharge and other flow information will be obtained from the USGS <br />from their Denver Basin groundwater model. <br />• The model development will include definition of aquifer boundaries. In most cases the <br />alluvial boundaries will correspond with the extent as presented in the Task 42 (Aquifer <br />Configuration) work products. However, in some locations, the modeled alluvial boundary <br />may differ from this based on the presence of wells, irrigation ditches or other features which <br />would better define the .alluvial boundaries for the purposes of numerical flow modeling. <br />Locations where the modeled alluvial boundary differs from the extent presented in Task 42 <br />work products and. the .rationale for moving the boundary will be discussed with the State. If <br />necessary, and following the State's approval, the Task 42 work products will be updated if <br />the budget allows or will be updated in a later phase. <br />Model simulations required to represent estimated historic pre-pumping conditions for the <br />alluvium will be undertaken. It is anticipated that this will represent early 1900's conditions. <br />The numerical model will be made operational by converging to specified head closure <br />criterion, and will have mass balance errors within acceptable limits. Both the head closure <br />SPDSS Groundwater SOW, Phase 3 1$ <br />