A soil-water retention function that includes the hyper-dry region through the BET adsorption isotherm



[1] Most existing full-range soil-water retention functions extend standard capillary pressure curves into the dry region to zero water content at a finite matric pressure. A description of dryness is commonly taken as oven-dry conditions given by a matric suction of about 109 Pa at zero liquid saturation. However, no finite pressure can be exerted by a zero amount of water, so a possibly more realistic situation necessarily implies that as water content approaches zero, suction tends to infinity. In this study we propose a full-range water retention function that takes advantage of the physical consistence of the Brunauer-Emmett-Teller (BET) adsorption isotherm to describe the very dry end, and preserves the capillary behavior of the classical Brooks and Corey function in the wet range. The transition from capillary to adsorption mechanisms is accounted for by a generalization of the Bradley's isotherm. Tests on seven widely studied soil data sets show that the experimental water retention curves are well fitted by the proposed retention model. In order to test the present approach, our simulations were compared to experimental data, for water transport under dry conditions, found in the literature. The present model was also compared with a recently proposed extended retention function in a hypothetical experiment designed to test the influence of predicted soil humidity on solute volatilization. These comparisons showed that, under severe dryness, the water dynamics is well described by the proposed model. Moreover, in these conditions the retention function determines the soil humidity, to which solute volatilization calculations can be very sensitive.