A modified Mualem-van Genuchten formulation for improved description of the hydraulic conductivity near saturation

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. 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 (h(s)) into the water retention curve. Analysis of the UNSODA soil hydraulic database revealed an optimal value of 24 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 240 cm were eliminated. A small correction was found necessary between 24 and 240 cm, and a much larger correction was needed between 0 and 24 cm. An average RMSE in logK 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.