title: Approaches to the simulation of unconfined flow and perched groundwater flow in MODFLOW.
abstract: Various approaches have been proposed to manage the nonlinearities associated with the unconfined flow equation and to simulate perched groundwater conditions using the MODFLOW family of codes. The approaches comprise a variety of numerical techniques to prevent dry cells from becoming inactive and to achieve a stable solution focused on formulations of the unconfined, partially-saturated, groundwater flow equation. Keeping dry cells active avoids a discontinuous head solution which in turn improves the effectiveness of parameter estimation software that relies on continuous derivatives. Most approaches implement an upstream weighting of intercell conductance and Newton-Raphson linearization to obtain robust convergence. In this study, several published approaches were implemented in a stepwise manner into MODFLOW for comparative analysis. First, a comparative analysis of the methods is presented using synthetic examples that create convergence issues or difficulty in handling perched conditions with the more common dry-cell simulation capabilities of MODFLOW. Next, a field-scale three-dimensional simulation is presented to examine the stability and performance of the discussed approaches in larger, practical, simulation settings.
authors.affiliation: S. S. Papadopulos and Associates, 7944 Wisconsin Ave., Bethesda, MD 20814, USA. vivekb@sspa.com
authors.forename: Vivek
authors.initials: V
authors.lastname: Bedekar
authors.forename: Richard G
authors.initials: RG
authors.lastname: Niswonger
authors.forename: Kenneth
authors.initials: K
authors.lastname: Kipp
authors.forename: Sorab
authors.initials: S
authors.lastname: Panday
authors.forename: Matthew
authors.initials: M
authors.lastname: Tonkin
mesh_terms: D003198:Computer Simulation; D060587:Groundwater; D012984:Software; D014872:Water Movements