Author
 |
GERKE, HORST - CTR FOR AGRIC. LANDSCAPE |
|
VAN GENUCHTEN, MARTINUS |
|
Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal Publication Acceptance Date: 4/16/1996 Publication Date: N/A Citation: N/A Interpretive Summary: Mathematical models are increasingly used to predict how agricultural chemicals move through the unsaturated zone between the soil surface and the groundwater table. Most existing models have been shown to perform reasonably well for relatively homogeneous soils. Unfortunately, most natural field soils are structured in some way by containing large pores between soil aggregates, drying cracks in clay soils, decayed root channels, worm holes, or other large macropores. Water and chemicals move through these soils much faster than can be predicted with existing computer models. This paper closely examines several models which may be used to predict the preferential movement of agricultural chemicals through macropores or other structural features. Of particular importance to this study is the exchange of water and chemicals between the macropores and the surrounding soil matrix. Equations are developed to account in a relatively simple manner for the geometry and size of the macropores or cracks and the matrix blocks on the rate of exchange of water and solutes between the macropores and the micropores of the soil matrix. Results of this study should be interest to scientists and engineers trying to develop improved management practices for agricultural chemicals. Technical Abstract: The structure of macroporous or aggregated soils and fractured rocks is generally so complex that it is impractical to measure the geometry at the microscale (i.e., the size and shape of soil aggregates or rock matrix blocks, and the myriad of fissures or fractures), and use such data in geometry-dependent macroscale flow and transport models. This paper analyzes a first-order type dual-porosity model which contains a geometry-dependent coefficient, b, in the mass transfer term to macroscopically represent the size and shape of soil or rock matrix blocks. As a reference, one- and two-dimensional geometry-based diffusion models were used to simulate mass transport into and out of porous blocks of defined shapes. Estimates for b were obtained analytically for four different matrix block geometries. Values for b were also calculated by directly matching analytical solutions of the diffusion models for a number of selected matrix block geometries, especially when relatively small ratios between the outer soil mantle and the radius of the inner cylinder were used. Results of our analysis show that b is closely related to the ratio of the effective surface area available for mass transfer, and the soil matrix volume normalized by the effective characteristic length of the matrix system. Using values of b obtained by direct matching, an empirical function is derived to estimate macroscopic geometry coefficients from medium properties which in principle are measurable. The method permits independent estimates of b, thus allowing the dual-porosity approach eventually to be applied to media with complex and mixed types of structural geometry. |
