A Process-Based Full-Range Model of Soil Water Retention.

Currently-used retention models vary in the degree to which their mathematical formulas correspond to controlling physical processes such as capillarity, adsorption, and air-trapping; such correspondence has often been a lower design priority than ease of use and compatibility with other property models. In particular the wettest range is normally represented simplistically, as by a straight line of zero slope, or by default using the same formulation as for the middle range.

The model proposed here recognizes dominant processes in three portions of the range from oven-dryness to saturation. It applies to soil matrix material, exclusive of macropores. The adsorption-dominated dry range is represented by a logarithmic relation used in earlier models. The middle range, of capillary advance/retreat and Haines jumps, is represented by a new adaptation of the lognormal distribution function. The wet range is dominated by trapped air expansion in response to pressure change, as well as a process that increases the sensitivity to changing matric pressure, which may be related in part to a collapse of liquid bridges in response to air expansion. For these the model uses the Boyles’ law inverse-proportionality of trapped air volume and pressure, amplified by an empirical factor to account for the additional process. The model’s eight parameters, though more than in most models, have a strong physical interpretation because they are process-based. Their values therefore can more readily be obtained from fundamental considerations or individual measurements, and have potential for systematic adjustment to account for hysteresis. Another substantial advantage is the physically-plausible treatment of the wet range, which avoids such problems as the blowing-up of derivatives on approach to saturation. This makes the model especially valuable for important but challenging wet-range phenomena such as domain exchange between preferential flow paths and soil matrix.