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United States Department of Agriculture

Agricultural Research Service

Title: Improved Description of the Hydraulic Properties of Unsaturated Structured Media Near Saturation

Authors
item Van Genuchten, Martinus
item Schaap, Marcel - UC RIVERSIDE, CA

Submitted to: Meeting Abstract
Publication Type: Proceedings
Publication Acceptance Date: December 31, 2003
Publication Date: February 1, 2004
Citation: Van Genuchten, M.T., Schaap, M.G. 2004. Improved description of the hydraulic properties of unsaturated structured media near saturation. In: Faybishenko, B., Witherspoon,P.A., editors. Proceedings of the 2nd Int. Symposium on "Dynamics of Fluids in Fractured Rock", Feb. 10-12, 2004, Berkeley, CA. p. 255-259.

Interpretive Summary: This paper focuses on the problem of preferential flow, a major challenge when dealing with flow and contaminant transport in the vadose zone. Preferential flow is caused by a broad range of processes. In structured or macroporous soils water may move through interaggregate pores, decayed root channels, earthworm burrows, and drying cracks. Similar processes occur in unsaturated fractured rock where water may move preferentially through fractures, thus bypassing much of the rock matrix. Preferential flow, as opposed to uniform flow, leads to irregular wetting patterns in the soil profile as a direct consequence of water moving faster in certain parts of the soil profile than in others. In this paper, we discuss various approaches for modeling preferential flow in the vadose zone between the soil surface and the groundwater table. Existing approaches range from relatively simplistic models to more complex physically based dual-porosity and dual-permeability models. A simple but effective approximation of preferential flow results when the traditional (Richards) equation is still used to predict unsaturated flow, but with composite (bimodal type) hydraulic conductivity curves that lump the effects of flow through the macropores and micropores into one single conductivity curve. Field data indicate that the macropore conductivity is generally about one order of magnitude larger than the matrix conductivity at saturation. Our analysis using a large unsaturated soil hydraulic database revealed similar differences between the macropore and matrix saturated hydraulic conductivities. We also found that a relatively simple function can be used to account for macropore flow close to saturation. Results are important to better understand and predict the fate and transport of agricultural contaminants in the subsurface.

Technical Abstract: Dual-porosity and dual-permeability models for preferential flow in unsaturated structured media (macroporous soils, fractured rock) generally assume that the medium consists of two interacting pore regions, one associated with the macropore or fracture network, and one with micropores inside soil aggregates or rock matrix blocks. A simple but effective approximation of preferential flow results when a single Richards equation is still used in an equivalent continuum approach, but with composite (bimodal type) hydraulic conductivity curves, rather than a single unimodal curve used in most traditional analyses. Field data indicate that the macropore conductivity is generally about one order of magnitude larger than the matrix conductivity at saturation. Neural-network analysis of the UNSODA unsaturated soil hydraulic database revealed a similar difference between the macropore and matrix saturated hydraulic conductivities. Further analysis of the database shows that a piece-wise log-linear function can be used for the macropore hydraulic conductivity between pressure heads of 0 and -40 cm. Results significantly improve the description of the hydraulic properties of structured field soils.

Last Modified: 11/21/2014
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