Modeling Soil Water Retention Curves in the Dry Range Using the Hygroscopic Water Content

Accurate information on the dry end (matric potential less than -1500 kPa) of soil water retention curves (SWRCs) is crucial for studying water vapor transport and evaporation in soils. The objectives of this study were to assess the potential of the Oswin model for describing the water adsorption curves of soils and to predict SWRCs at the dry end using the hygroscopic water content at a relative humidity of 50% (theta(RH50)). The Oswin model yielded satisfactory fits to dry-end SWRCs for soils dominated by both 2:1 and 1:1 clay minerals. Compared with the Oswin model, the Campbell and Shiozawa model combined with the Kelvin equation (CS-K) produced better fits to dry-end SWRCs of soils dominated by 2:1 clays but provided poor fits for soils dominated by 1:1 clays. The shape parameter a of the Oswin model was dependent on clay mineral type, and approximate values of 0.29 and 0.57 were obtained for soils dominated by 2:1 and 1:1 clays, respectively. Comparison of the Oswin model combined with the Kelvin equation, with water potential estimated from theta(RH50) (Oswin-K-RH50), CS model combined with the Arthur equation (CS-A), and CS-K model, with water potential obtained from theta(RH50) (CS-K-RH50) indicated that for soils dominated by 2:1 clay minerals, the predictive ability of the Oswin-K-RH50 model was comparable to the CS-K-RH50 model in which theta(RH50) was the input parameter but performed better than the CS-A model where clay content was the input parameter. The Oswin-K-RH50 model also has the potential for predicting dry-end SWRCs of soils dominated by 1:1 clays.