Three-dimensional volume-averaged soil moisture transport model with a scalable parameterization of subgrid topographic variability

[1] Subgrid variability of subsurface moisture flux transport is strongly influenced by the local variation of topographic attributes, such as elevation, slope, and curvature. A three-dimensional volume-averaged soil moisture transport (VAST) model is developed to incorporate these effects using the volume-averaged Richards equation. The small-perturbation approach is used to decompose the equation into mean and fluctuation, which are then averaged over the model grid box. This formulation explicitly incorporates the variability of moisture flux due to subgrid variation of topographic attributes. The model is independent of scale, but the parameters need to be estimated at the model scale. It is demonstrated that the flux contribution from the subgrid variability can be comparable to that of mean flux, particularly under drier moisture conditions. This formulation can be substituted for subsurface moisture transport schemes in most existing land surface models.