A New StreamflowRouting (SFR1) Package to Simulate StreamAquifer Interaction with MODFLOW2000By David E. Prudic, Leonard F. Konikow, and Edward R. Banta Report availability: Portable Document Format (PDF). 

AbstractThe increasing concern for water and its quality require improved methods to evaluate the interaction between streams and aquifers and the strong influence that streams can have on the flow and transport of contaminants through many aquifers. For this reason, a new StreamflowRouting (SFR1) Package was written for use with the U.S. Geological Survey's MODFLOW2000 groundwater flow model. The SFR1 Package is linked to the Lake (LAK3) Package, and both have been integrated with the GroundWater Transport (GWT) Process of MODFLOW2000 (MODFLOWGWT). SFR1 replaces the previous Stream (STR1) Package, with the most important difference being that stream depth is computed at the midpoint of each reach instead of at the beginning of each reach, as was done in the original Stream Package. This approach allows for the addition and subtraction of water from runoff, precipitation, and evapotranspiration within each reach. Because the SFR1 Package computes stream depth differently than that for the original package, a different name was used to distinguish it from the original Stream (STR1) Package. The SFR1 Package has five options for simulating stream depth and four options for computing diversions from a stream. The options for computing stream depth are: a specified value; Manning's equation (using a wide rectangular channel or an eightpoint cross section); a power equation; or a table of values that relate flow to depth and width. Each stream segment can have a different option. Outflow from lakes can be computed using the same options. Because the wetted perimeter is computed for the eightpoint cross section and width is computed for the power equation and table of values, the streambed conductance term no longer needs to be calculated externally whenever the area of streambed changes as a function of flow. The concentration of solute is computed in a stream network when MODFLOWGWT is used in conjunction with the SFR1 Package. The concentration of a solute in a stream reach is based on a massbalance approach and accounts for exchanges with (inputs from or losses to) groundwater systems. Two test examples are used to illustrate some of the capabilities of the SFR1 Package. The first test simulation was designed to illustrate how pumping of ground water from an aquifer connected to streams can affect streamflow, depth, width, and streambed conductance using the different options. The second test simulation was designed to illustrate solute transport through interconnected lakes, streams, and aquifers. Because of the need to examine time series results from the model simulations, the Gage Package first described in the LAK3 documentation was revised to include time series results of selected variables (streamflows, stream depth and width, streambed conductance, solute concentrations, and solute loads) for specified stream reaches. The massbalance or continuity approach for routing flow and solutes through a stream network may not be applicable for all interactions between streams and aquifers. The SFR1 Package is best suited for modeling longterm changes (months to hundreds of years) in groundwater flow and solute concentrations using averaged flows in streams. The Package is not recommended for modeling the transient exchange of water between streams and aquifers when the objective is to examine shortterm (minutes to days) effects caused by rapidly changing streamflows.  
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