Evaluation of multimodal hydraulic functions in characterizing a heterogeneous field soil

Soil water retention curves are often used to estimate the hydraulic conductivity function. Unfortunately, single S-shaped functions cannot adequately describe water retention curves of structured soil, especially near saturation. The approach of superposition of two or more unimodal retention functions such as the van Genuchten model was used here to describe retention data of a macroporous soil. A total of 180 cores, 0.05 m diameter and 0.051 m long, were sampled along a 31-m-long transect in three overlying soil horizons. Use of unimodal retention curves leads to an underestimation of observed water contents both near saturation and in the midpore range, while an overestimation is found in the drier range. Superposition of two unimodal retention curves significantly improved the estimation over the entire pressure range. However, the predictions were still not ideal near saturation. With three unimodal curves, a perfect fit was obtained from saturation to residual water content. Most of the multimodal parameter values were moderately heterogeneous along the transect, with the surface horizon slightly more heterogeneous than the deeper layers. The coefficient of variation (CV) for multimodal parameters was generally in the range of 20 to 70%. Use of the multimodal van Genuchten model with the conductivity estimation model of Mualem resulted in conductivities that were generally much smaller than those estimated by the classical unimodal van Genuchten-Mualem model. A preliminary evaluation of the estimated bimodal and trimodal unsaturated hydraulic conductivity model was based on a comparison with independent conductivity measurements using a combination of crust test, hot-air method, and an unsteady drainage flux experiment on large columns. The crust and hot-air data compared best with the estimated trimodal conductivity function. The unsteady drainage data did not match well with the crust and hot-air data and could not be described with any of the estimated conductivity functions.