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 -4 cm for h_s, 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 eliminated. A small correction was found necessary between -4 and -40 cm, and a much larger correction was needed between 0 and -4 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.