A MODIFIED MUALEM-VAN GENUCHTEN FORMULATION FOR IMPROVED DESCRIPTION OF THE HYDRAULIC CONDUCTIVITY NEAR SATURATION

Authors: SCHAAP, MARCEL - UC RIVERSIDE, CA; Van Genuchten, Martinus

Submitted to: Vadose Zone Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/11/2005
Publication Date: 12/16/2005
Citation: Schaap, M.G., Van Genuchten, M.T. 2005. A modified mualem-van genuchten formulation for improved description of the hydraulic conductivity near saturation. Vadose Zone Journal. Vol 5:27-34

Interpretive Summary: Computer models are increasingly used in research and management to predict the movement of water and dissolved chemicals in the unsaturated zone between the soil surface and the groundwater table. Effective use of these models requires accurate estimates of the unsaturated hydraulic conductivity or permeability, K, which governs the rate at which water moves through the soil profile. This paper addresses the shape of a popularly used theoretical equation for the conductivity as a function of the degree of saturation (or soil water content). Recent studies suggest several shortcomings of this equation, especially its inability to account for the accelerated flow of water through macropores when the soil is close to being saturated with water. In this study we present a modified formulation that improves the description of the hydraulic conductivity. The modified model may be viewed as accounting for flow through separate micropores (the soil matrix) and macropores (fractures), possibly augmented with flow through intermediate sized mesopores. Using data from a large experimental database, we show that the new formulation provides a substantial improvement in the macroscopic description of the hydraulic conductivity function. The model seems to be especially suited for large-scale studies that require realistic simulations of saturated or near-saturated infiltration into soils. Results of this study are pertinent for modelers and engineers trying to predict or manage the movement of water and agricultural chemicals in soils.

Technical Abstract: The unsaturated soil hydraulic properties are often described using Mualem-van Genuchten (MVG) type analytical functions. Recent studies suggest several shortcomings of these functions near saturation, notably the lack of second-order continuity of the soil water retention function at saturation, and the inability of the hydraulic conductivity function to account for macroporosity. In this study we present a modified MVG formulation that improves the description of the hydraulic conductivity near saturation. The modified model introduces a small but constant air entry pressure (hs) into the water retention curve. Analysis of the UNSODA soil hydraulic database revealed an optimal value of -4 cm for hs, more or less independent of soil texture. The modified model uses a pressure dependent piece-wise linear correction to ensure that deviations between measured and fitted conductivities between pressure heads of 0 and -40 cm were liminated. A small correction was found necessary between -4 and -40 cm, and a much larger correction between 0 and -4 cm. An average root mean square error in log K of only 0.26 remained for a data set of 235 samples. The resulting modified MVG model was found to have small systematic errors across the entire pressure range. The modified model appears well suited for large-scale vadose zone flow and transport simulations, including inverse modeling studies.