An Integrated Approach for Modeling Water Flow and Solute Transport in the Vadose Zone

As geographical information systems (GIS) are increasingly being applied to surface and subsurface flow and transport modeling issues, it becomes important to more clearly define potential advantages and achievable objectives with this technology. This chapter describes an integrated conceptual framework for predicting basin-scale solute loading rates through and from the vadose zone. The approach conceptually couples the ARC/INFO geographical information system with a deterministic variably-saturated flow and transport model (HYDRUS), an unsaturated soil hydraulic property database (UNSODA), a digital soil database (STATSGO) in conjunction with pedo-transfer functions (PTFs), and a geostatistical software package (GEOPACK). Suggestions are made on how best to integrate currently available or future knowledge of surface hydrology, vadose zone hydrology, and groundwater hydrology so as to more effectively address specific nonpoint source pollution problems. Whereas computations by the different components within the proposed integrated architecture can be made to run interactively, individual components will keep their identities at previously defined positions. The resulting integrated approach involving loosely-coupled independent technologies should provide new possibilities for addressing non-point source subsurface pollution problems. Quantifying and modeling subsurface non-point source pollution is being viewed by environmental professionals as an important, yet often frustrating problem in view of the overwhelming heterogeneity of the subsurface environment. The intrinsic nonlinear nature of flow and transport processes in variably-saturated media makes the problem very difficult to describe and treat mathematically. Traditional process-based and more recent probabilistic approaches for addressing non-point source pollution are reviewed by van Genuchten (1994) and Jury (1996, this publication), respectively. Even if the basic physical, chemical, and biological processes operating in the subsurface could be properly quantified in terms of mathematical equations (usually partial differential equations), and subsequently solved accurately and efficiently, spatial and temporal heterogeneity of the three-dimensional soil profile would make the problem still relatively unmanageable. Accurate description and monitoring of the spatio-temporal variability Copyright