Wednesday, May 4, 2011: 3:10 p.m.
Annapolis (Hyatt Regency Baltimore on the Inner Harbor)
Vivek Bedekar, S.S. Papadopulos and Associates;
Sorab Panday, AMEC Geomatrix Inc.;
Matthew J. Tonkin, S.S. Papadopulos and Associates Inc.;
Various approaches have been proposed to deal with the non-linearities associated with the unconfined flow equation in MODFLOW. Doherty (2001), Painter et al. (2008), Panday and Huyakorn (2008), HydroGeoLogic (1998), and Niswonger et al. (2010, in press) discuss various numerical techniques to achieve a stable solution for various formulations of the unconfined groundwater flow equation developed to prevent dry cells from becoming inactive thereby avoiding discontinuous head solution at “dry” cells. Most of these approaches implement an upstream weighting formulation and Newton-Raphson linearization to obtain robust convergence. In this study, all of the suggested approaches were implemented in a stepwise manner into MODFLOW-2000 (Harbaugh et al. 2000) for comparative analysis and to further experiment with these, and other, schemes. To accommodate the asymmetric coefficient matrix generated by the Newton-Raphson scheme, an ORTHOMIN solver was implemented. A comparative analysis is presented using synthetic examples which otherwise create convergence issues with the more common dry-cell simulation capabilities of MODFLOW. A field-scale three-dimensional simulation is also presented.
References
Doherty, J., 2001, Improved calculations for dewatered cells in MODFLOW. Ground Water 39(6): 863–869.
Harbaugh, A.W., Banta, E.R., Hill, M.C., and McDonald, M.G., 2000, MODFLOW-2000, the U.S. Geological Survey modular ground-water model -- User guide to modularization concepts and the Ground-Water Flow Process: USGS Open-File Report 00-92, 121 p.
HydroGeoLogic, 1998. MODFLOW-SURFACT v. 3.0: A comprehensive MODFLOW-based flow and transport simulator. HydroGeoLogic, Reston, VA.
Niswonger, R.G., Sorab Panday, and Motomu Ibaraki, in press, Upstream Weighting Package and Newton Solver Package for MODFLOW-2005: USGS Techniques and Methods Book 6.
Painter, S., H. Basagaoglu and A.G. Liu. “Robust Representation of Dry Cells in Single-Layer MODFLOW Models.” Ground Water, No. 46, (2008): 873–881
Panday, S., and P.S. Huyakorn. 2008. MODFLOW-SURFACT: A state-of-the-art use of vadose zone flow and transport equations and numerical techniques for environmental evaluations. Vadose Zone J. 7:610–631
