Improved Calculations for Dewatered Cells in MODFLOW

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Abstract
Use of the United States Geological Survey ground water flow model MODFLOW is often hampered by the occurrence of “dry cells.” While MODFLOW allows such cells to “rewet” in the course of a simulation, stability of the heads solution process is often problematical with rewetting functionality operative. In many cases of practical interest (particularly in mining applications), MOD‐FLOW simply fails to converge. However by making a number of adjustments to the MODFLOW Block‐Centered Flow package, it is possible to overcome this problem in many instances of MODFLOW deployment. These adjustments are such as to allow a layer to transmit water, albeit with a vastly reduced transmissivity, even if the water level in that layer is below its base. With these alterations MODFLOW cells can remain active even if they lie within the unsaturated zone. Testing of the code has demonstrated its ability to perform well in situations where performance of the unmodified MODFLOW is degraded by the necessity to dry and rewet cells. Comparison of heads calculated using the modified MODFLOW with those calculated using MODFLOW‐SURFACT (a MODFLOW‐based code developed by HydroGeoLogic Inc. that prevents the occurrence of dry cells through use of pseudo soil functions) reveals near‐identical results between the two codes. Comparison with analytical solutions of water table location also reveals near coincidence. An example of one such application is presented herein.