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SPDSS Groundwater Technical Peer Review Committee Meeting Minutes <br />February 13, 2007 <br />Page 8 <br />a. Inflow -recharge from precipitation and irrigation; stream losses to aquifers. Recharge on: <br />non-irrigated areas 0.2 to 1.3 in/yr (varies spatially); alluvial aquifers 2.5 to 3.6 in/yr; urban <br />irrigation; agricultural irrigation. Recharge from precipitation is a calibration parameter; recharge <br />from irrigation is an input parameter. <br />b. Outflow -discharge to springs and streams; evapotranspiration; pumping. Natural discharge - <br />springs, streams, evapotranspiration. Evapotranspiration -maximum rate 45 in/yr. Represents <br />ET rate as function of depth to water (as for RGDSS). Extinction depth 13 feet. <br />4. Model calibration - Compare model-computed water levels/flows to measured values. Adjust model <br />input parameters within reasonable ranges until model-computed and observed values match. <br />5. Denver Basin observations - Water level calibration data 1200+ sites, approximately 5,000 water level <br />observations (1880 - 2003). Pumping rates consistent with SB-74 model. Streamflow data from <br />baseflow analysis. Pumping estimate details presented -domestic/municipal/commercial wells, alluvial <br />vs. bedrock wells, etc. <br />D. Model use <br />1. Can compute head elevation and drawdown over simulation period. <br />2. Understanding changes in water budget: Recharge and ET important/largest components. 2003 <br />pumping was approximately 25% of all outflows. Pumping is reducing storage and flow to streams and <br />alluvium. <br />3. Questions model can help to answer: How long for water to move from recharge areas to wells? How <br />does travel time compare to estimated groundwater age? Where/what data needed to improve model <br />calibration and predictions? What are optimal pumping rates/locations to minimize drawdown and <br />storage depletions? <br />4. Predictions model can make -How will water levels and water budget respond to: increases in pumping <br />and geographic spread of pumping in response to population growth/drought; decreases in pumping as <br />systems change to surface water supplies. <br />E. Discussion <br />1. Goal for model completion/use -October 2007. <br />Question: How are Division 2 alluvial systems being determined/what are the inputs? <br />S. Paschke Response: Alluvial aquifer is explicitly represented in the model as unconfined aquifer in <br />Layer 1. Layer 1 extent and thickness taken from SPDSS and USGS efforts (previous mapping of extent <br />and thickness). Average hydraulic conductivity values are used -one value for the mainstem of the <br />South Platte River and another value for the lesser tributaries. We are thinking of the alluvial aquifers as <br />a boundary condition to the bedrock aquifers. The bedrock model does not contain as detailed a <br />representation of the alluvial aquifer as will the SPDSS alluvial aquifer model. <br />3. S. Paschke clarified that K and S are model "parameters" that represent sand fraction of aquifers; model <br />K's not directly related to point data. <br />