Can fill-and-spill subsurface flow be represented by a moisture-dependent anisotropic permeability tensor in Richards equation-based models with coarse spatial resolution?

Abstract:

The fill-and-spill hypothesis has been proposed to explain threshold runoff responses to precipitation at the hillslope scale. Fill-and-spill suggests lateral subsurface flow paths are activated once depressions in the microtopography at the soil-bedrock interface become saturated and connect laterally. However, Richards equation-based modeling can only reproduce fill-and-spill with very detailed subsurface information, including the microtopography of the soil-bedrock interface. Richards equation-based model with coarse spatial resolution and effective parameters cannot reproduce fill-and-spill runoff generation.

We propose that Richards equation-based models equipped with a moisture-dependent anisotropic permeability at the soil-bedrock interface may be able to reproduce the fill-and-spill phenomenon even with a coarse spatial resolution. By activating the lateral anisotropy under wet conditions, this model can generate lateral flow, and so reproduce the threshold runoff response.

To test this idea, we used ParFlow to model virtual hillslopes with synthetic rugged internal topography. We then investigated the effective permeability tensor over an averaging volume that encloses the interface. By parameterizing the anisotropy of an interface block under changing moisture variables, we may be able to generate predictions of how the fine-scale fill-and-spill processes integrate to the hillslope and the catchment scales.