The soil zone has been described as the upper most region of the vadose zone where plant and soil processes enhance storage and permeability, providing a fast pathway for water and solutes to streams. The soil zone connects the land surface to the deeper unsaturated and saturated zones. Modeling of soil-zone processes has been used to gain understanding of watershed hydrologic processes. Currently MODFLOW does not simulate dynamic near-surface hydrologic processes such as, infiltration, hortonian runoff, dunnian runoff, and return flow. The Soil-Zone Flow Package (SZF) for MODFLOW is being developed to address these near-surface components for simulating watershed processes in the context of basin-scale groundwater-flow modeling, especially those processes that partition rainfall into evapotranspiration, runoff, and deep percolation.

In a series of test simulations, Richards' equation (RE) was compared with MODFLOW-SZF. Across a range of hydraulic conductivities and applied precipitation rates, MODFLOW-SZF achieved a good infiltration and interflow solution (0.014-0.052 RMSE) with only 6 grid cells and 0.11 seconds computation time compared to the 6250 grid cells and 40 seconds of computation time required for a stable solution to RE. MODFLOW-SZF solutions had negligible errors due to grid effects (<0.01 RMSE) however, RE solutions were sensitive to grid resolution (up to 0.12 RMSE). Errors associated with using the groundwater flow equation to represent soil-zone flow instead of RE were much lower (<0.052 RMSE) than the errors caused by grid effects in RE (0.12 RMSE, 0.5 m cell). This research suggests that the SZF Package will be an effective tool for efficiently representing soil-zone processes at the basin scale.