Quantifying water adsorption ability of unsaturated soil by soil water isotherm

Adsorption plays a crucial role in governing soil water retention and significantly effects the hydromechanical behavior of unsaturated soil, which is, however, often overlooked. This study proposes that a thorough evaluation of soil adsorption ability should be undertaken by considering two perspectives: the specific surface area (SSA) and adsorption strength per unit area (soil sorptive potential, SSP). To achieve this, a soil water isotherm (SWI)-based method is developed to determine them, which considers the temperature effects and allows SWI measured at arbitrary temperatures as input. It solves the SSA and SSP coupling from the SWI, avoiding inaccurate SSA results caused by ignoring SSP. Experimental and simulated data validate the generality and accuracy of the method. The analyses reveal that sorption energy plays a pivotal role in the spatial variation and temperature dependence of SSP. Moreover, it is evident that SSA and SSP are independent quantities that can describe the soil adsorption ability and determine the adsorbed water content together. In comparison with existing methods, the proposed method exhibits higher accuracy, requires fewer fitting parameters, and is applicable across a wider temperature range. This study provides a new approach for comprehensively assessing the adsorption ability of soil under various environmental conditions.