Implementation and Evaluation of Fully 3-D Richards’ Equation in MODFLOW 6

TRRP Training: 2022 Program

presented by: GSI Environmetal Inc.

Texas Risk Reduction Program regulations (TRRP; 30 TAC 350) establish consistent risk-based protocols for assessment and response to soil, groundwater, or surface water impacts associated with environmental releases of regulated wastes or substances.

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Dates

June 14th and 15th, 2022

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Crowne Plaza River Oaks 2712 SW Freeway Houston, Texas 77098 713.523.8448 http://www.crowneplaza.com/

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Download the Richards' equation implementation for MODFLOW 6 and the complete input files for the three benchmark problems presented in the paper.

This page accompanies the paper Evaluation and Implementation of Fully 3-D Richards’ Equation in MODFLOW 6, by Sebastián Vázquez-Gasty, Sorab Panday, Christian Langevin, and Tirthankar Roy — a collaboration between GSI Environmental, S.S. Papadopulos & Associates and the University of Nebraska-Lincoln.

The work implements a fully three-dimensional Richards’ equation solver within the MODFLOW 6 framework, so that the unsaturated zone and the groundwater system are represented together in a single model. The formulation uses the mixed form of Richards’ equation with van Genuchten constitutive relations and Newton-Raphson linearization.

The unsaturated-zone approaches currently available in MODFLOW neglect capillarity and treat unsaturated flow as one-dimensional; resolving capillary-driven flow in all three dimensions allows processes such as lateral moisture redistribution and perched water above low-permeability layers to be simulated directly.

The implementation was benchmarked against four established variably saturated flow codes — HYDRUS, MODFLOW-VSF, MODFLOW-USG Transport and SUTRA — on three problems: a layered one-dimensional soil column, a sloping three-dimensional agricultural field under a year of daily recharge, and a heterogeneous version of that field containing a low-permeability lens that produces perched water. Full results and discussion are in the paper: [Vázquez-Gasty et al., journal, year, DOI (This will need to be uptaded after the paper is published)].

The link above contains a ZIP file with the MODFLOW 6 Richards’ equation implementation together with the complete input files for all three benchmark problems, so that the published results can be reproduced and the same problems can be solved with other codes for comparison.

This is a research implementation made available to accompany the paper. It is not part of an official U.S. Geological Survey release of MODFLOW 6, and the formulation ultimately adopted in any future official release may differ from the description provided in the paper. Questions about the implementation or the benchmark problems can be directed to Sebastián Vázquez-Gasty.

Simulated saturation from the MODFLOW 6 Richards’ equation implementation for the three-dimensional benchmark problem: a cross-section through the sloping field showing the unsaturated zone above the water table, and the saturation history at a monitoring point where MODFLOW 6, USG-T, MODFLOW-VSF and SUTRA agree.