Performance of hydraulic conductivity models for use in numerical simulations of water flow in unsaturated soils

Numerical solvers of the partial differential equation for soil water flow (Richards’ equation) require the soil water retention curve (SWRC) and the unsaturated soil hydraulic conductivity curve (UHCC). Recent UHCC models included water flow in films in addition to capillary flow to improve the description of the dry-range conductivity. Here, we consider an established UHCC model with capillary flow only, an additive model that sums conductivities for water in films and capillaries, and a junction model that has only film flow in the dry range and only capillary flow in the wet range. We tested the effect of the UHCC models and different combinations of fitted and fixed model parameters on water fluxes calculated by Hydrus-1D, a popular Richards’ solver. All UHCC models were used in conjunction with a recent SWRC model with physically sound behavior in the dry range and at saturation. The test focused on fluxes across the soil surface and at 2 m depth, calculated for three uniform soils subjected to three climates. All three UHCC models fitted measured conductivities well, even though the capillary-only and the junction model had one parameter less than the additive model. The calculated fluxes were in general agreement, but Hydrus crashed more often for the additive model than for the other two. The effect of film flow was small. Calculations based on different sets of fitting parameters showed that often, not all parameters need to be fitted. This favors the more parsimonious and robust capillary-only and junction models.