Submitted to: Journal of Hydrology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/9/2012
Publication Date: 5/18/2012
Citation: Kuznetsov, M., Yakirevich, A., Pachepsky, Y.A., Sorek, S. 2012. Quasi 3D modeling of water flow in vadose zone and groundwater. Journal of Hydrology. 450-451:140-149. Interpretive Summary: Modeling water flow in unsaturated soils is extremely time-consuming and difficult because of the complexity of parameters characterizing the ability of unsaturated soils to retain and transmit water. In particular, it is difficult to determine these parameters for deep soil layers. As a consequence, it is often deemed preferable and acceptable to assume that lateral water flow in unsaturated soils is not significant. However this assumption can result in errors in modeling water flow through the unsaturated zone and at the surface of groundwater. We proposed a new modeling method that uses the same system of equations for both saturated and unsaturated zones and preserves water fluxes at groundwater surface. The computational accuracy of the method has been demonstrated with data from laboratory experiment and with simulations. The method is implemented in the USGS MODFLOW modeling system. Results of this work will be useful for environmental scientists and modelers in that the method provides for more efficient simulations of water flow in variably saturated soils and allows one to evaluate the potential errors that may arise when lateral flow in unsaturated soil is ignored.
Technical Abstract: The complexity of subsurface flow systems calls for a variety of concepts leading to the multiplicity of simplified flow models. One habitual simplification is based on the assumption that lateral flow and transport in unsaturated zone are not significant unless the capillary fringe is involved. In such cases the flow and transport in the unsaturated zone above groundwater level can be simulated as a 1D phenomenon, whereas the flow and transport through groundwater are viewed as 2D or 3D phenomena. A new approach for a numerical scheme for 3D variably saturated flow using quasi 3D Richards’ equation and finite difference scheme’ is presented. The corresponding numerical algorithm and the QUASI-3D computer code were developed. Results of the groundwater level simulations were compared with transient laboratory experimental data for 2D data constant–flux infiltration, quasi 3D HYDRUS-MODFLOW numerical model and a FULL-3D numerical model using Richard’s equation. Infiltration and pumping hypothetical 3D examples for three different spatial-time scales and two porous media are presented. Computationally the QUASI-3D was more efficient by an order of 10% to 300%, while being accurate in comparisons with the benchmark fully 3D variable saturation code, when the capillary fringe was considered.