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Method and computer programs to improve pathline resolution near weak sinks representing wells in Modflow and Modpath ground-water-flow simulations

Open-File Report 00-392

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      This report documents a method for converting weak sinks representing wells to strong sinks when using the particle-tracking postprocessor MODPATH for the ground-water-flow model MODFLOW. A weak sink is a model cell (representing a well, for example) that does not discharge at a sufficiently large rate to capture all of the flow entering the cell; thus, some of the flow leaves the cell across one or more of the cell faces. Because of this limitation of model discretization, flow paths to weak sink cells cannot be uniquely defined, as it is impossible to know whether a specific water particle discharges to the sink or passes through the cell. Creating a finer discretization of the cell by using the nested rediscretization method can eliminate this ambiguity by converting the cell representing the well into a strong sink. The method is presently limited to applications involving well sinks, backtracking particle endpoint analysis, and steady-state conditions. Software for applying the method consists of five Fortran programs that are operated manually. Program input is a combination of data files and interactive keyboard entry. Program output consists of a composite endpoint file. Application of the method to a sample problem demonstrates that the particle-tracking results obtained can be more accurate than those obtained by using a coarsely discretized model alone. This method is a relatively simple alternative to more involved telescopic mesh refinement approaches and does not require additional information. For certain types of problems involving identification of recharge source areas, this method can provide improved pathline resolution with modest effort. This method could be adapted for use in simulating other types of weak sinks, such as stream cells, but each type of sink boundary has unique considerations that may have to be treated differently than those described in this report.

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