Unimodal and bimodal descriptions of hydraulic properties for aggregated soils

This study was conducted to investigate the capability of bimodal approaches in describing water retention data and predicting hydraulic conductivity of 18 samples from an aggregated soil. In this soil, discontinuity in the shape of the water retention curve was encountered and explained by independent draining of the inter- and of intraaggregate pores. A single van Genuchten-type retention curve was unable to describe the observed transition between the pore systems, especially near saturation. Such behavior occurred both when optimizing (VGopt) and when fixing at the measured value (VGfix) the volumetric water content at saturation, theta(s). Because of the predominant effect of the shape of the retention curve near saturation upon the shape of the whole hydraulic conductivity curve, predictions of hydraulic conductivity often differed from unsaturated conductivity observations by even two orders of magnitude. To the contrary, excellent descriptions of retention data were observed when superposition of two unimodal retention curves was adopted. The first function was either a van Genuchten (VGbim) or a simple one-parameter formulation introduced by Boss and Smettem (RSbin) for describing macroporosity, while the second was in both cases a van Genuchten formulation. The agreement was especially good for higher water content values, leading to values of the coefficient of determination, R-2, very close to unity. The laboratory-measured unsaturated conductivity values compared more closely when bimodal approaches were used, and the predictions were frequently well within one order of magnitude of the measurements.