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To demonstrate its utility in representing realistic vadose zone flow, the MODFLOW-WHT model was used to simulate the infiltration study of Warrick et al (1971).
This field experiment was performed in a 6.1 meter^2 soil plot containing tensiometers at 30, 60, 90, 120, 150, 180 cm depths. A conservative tracer solution (0.2 N CaCl2) was used to track solute transport through the system. Measurements were collected for 17.5 hours during which the soil surface was kept at constant saturation.
The tracer solution was applied to the surface for 2.8 hours followed by 15.3 hours of water. The flow behavior was simulated with the model and the solute transport was simulated using a modified version of the RT3D transport model (Clement, 1997). The flow and transport results were compared with both the experimental results of Warrick et al. (1971) and the 1-D flow and transport model SUMATRA (van Genuchten, 1978).
The initial soil moisture profile is approximately linear from 0.15 (cm^3/cm^3) at the surface to 0.2 (cm^3/cm^3) at z equal to or less than 60 cm. The soil characteristic functions were fit to the soil hydraulic conductivity and capillary saturation data. A summary of the soil parameters and model conditions used for this study are listed in table 1. Results for the flow simulation after 2 hours and 9 hours of infiltration are shown in figure 1 plotted against the experimental data as well as simulation results from the 1-D unsaturated flow finite element model, SUMATRA (van Genuchten, 1978). The model represents the experimental results well; there is some over-prediction of the infiltration front at t=2 hours that can be attributed to the over-prediction of kr at low theta in the van Genuchten relative permeability relationship.
The tracer solution was applied to the surface for 2.8 hours followed by 15.3 hours of water. The concentration profiles after 2 hours and 9 hours of infiltration are shown in figure 2 compared against both the experimental data and the SUMATRA model (van Genuchten, 1978). The basic behavior of the solute in the unsaturated soil is represented satisfactorily, demonstrating proper implementation of the unsaturated transport equation.
van Genuchten, M. Th. Mass transport in saturated-unsaturated media: One-dimensional solutions, Research Report 78-WR-11, Princeton Univ., Princeton, NJ, 102 p., 1978.
Warrick, A.W., J.W. Biggar and D.R. Nielsen. Simultaneous solute and water transfer for an unsaturated soil. Water Resour. Res. 7:1216-1225, 1971.
Thoms, R.B. Simulating fully-coupled overland flow and variably saturated subsurface flow using MODFLOW, Masters Thesis, OGI School of Science and Engineering at OHSU, 136 p., 2003.
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