Robust Representation of Dry Cells in MODFLOW

Author SL Painter and H Basagaoglu
Year 2007
Description Documentation and testing of MODFLOW-NR to improve method for dealing with dry cells in modeling the Edwards Aquifer
Report Number 20-13003
Publisher Southwest Research Institute®
Location Edwards Aquifer, Balcones Fault Zone, San Antonio Segment
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When MODFLOW calculates a water level that is below the base elevation of a computational cell, that cell is declared to be dry and removed (temporarily or permanently) from the calculation, causing the model to produce unreliable results.  This problem with the MODFLOW system is well-known and long-standing.  The dry-cell issue was a significant obstacle to modeling water management scenarios with the Edwards Aquifer model because cells in the recharge zone often went dry.  This report describes a new variant of MODFLOW, denoted MODFLOW-NR, that incorporates a recently developed robust computational scheme for representing dry cells.  The new representation of dry cells combines an upstream-weighting algorithm for intercell conductances with a new solver package.  This report summarizes the capabilities, technical basis, data input requirements, software validation/verification tests, and example simulations for the MODFLOW-NR software.  The study was completed in May 2007 by Southwest research Institute®, San Antonio, Texas.

The combination of upstream weighting of intercell conductances with a Newton-Raphson solver avoids numerical instabilities and artifacts associated with dry cells. Tests performed to date using the MODFLOW-NR code indicate that the approach is very robust. Comparisons with the unmodified MODFLOW-2000 code demonstrate that the upstream weighting and conventional spatial differencing approaches produce very similar results.

Version 1.0 of MODFLOW-NR is limited to single-layer aquifers. However, the approach is applicable to aquifers with multiple layers. Fully three-dimensional capabilities could be added to future versions with a reasonable level of effort.

MODFLOW-NR has been tested with the Well, Drain, Recharge, River and Horizontal Flow Barrier packages. The recently developed Well Pumping Management Package (WMP1) is not included in Version-1 of MODFLOW-NR, but could be incorporated in future versions.