Soil Specific Surface Area and Non‐Singularity of Soil‐Water Retention at Low Saturations

The dry end of the soil water characteristic (SWC) is important for modeling vapor flow dynamics and predicting soil properties such as specific surface area (SSA) and clay content (CL). Verification of new instrumentation for rapid measurement of the dry end of the SWC is relevant to avoid long equilibration times and potential for hydraulic decoupling. The objectives of this study were to measure both adsorption and desorption branches of the dry end of the SWC for 21 variably‐textured Arizona soils using new, fully automated instrumentation (AquaSorp); apply the data to parameterize the Tuller and Or (TO) and new single‐parameter non‐singularity (SPN) models; and evaluate estimates of SSA from water sorption, ethylene glycol monoethyl ether (EGME), and N 2 –BET methods. The AquaSorp successfully measured water sorption isotherms (∼140 data points) within a reasonably short time (1–3 d). The SPN model well described the distinct non‐singularity between the adsorption and desorption branches, while the TO model captured the adsorption data reasonably well (<5% deviation from measurements), except for matric potentials below –200 MPa. The SSA derived from water sorption and the TO model were comparable to SSA EGME for all soils. The matric potential at “zero” water content was confirmed as the widely accepted value of around –800 MPa. A non‐singularity coefficient based on water adsorption at monolayer coverage was positively correlated with CL. Obtained results show the potential of the AquaSorp to accurately measure the dry region of the SWC, providing a rapid determination of SSA.