A state-dependent parameterization of saturated-unsaturated zone interaction

The relevance of groundwater as an important source of root zone moisture by means of capillary rise is increasingly being recognized. This is partly reflected in many current land surface schemes, which increasingly replace a one-way (i.e., downward) drainage of water by a two-way interaction flux between the root zone and a groundwater system. A fully physically correct implementation of this two-way saturated-unsaturated interaction flux requires transient simulations using the highly nonlinear Richards' equation, which is a computationally demanding approach. We test a classic simple approximation that computes the root zone-groundwater interaction flux as the net effect of a downward drainage flux and an upward capillary rise flux against the Darcy equation for quasi steady state conditions. We find that for a wet root zone and/or shallow groundwater, the errors within this approximation are significant and of the same magnitude as the interaction flux itself. We present a new closed-form parameterization of the Darcy equation-based fluxes that accounts both for root zone soil moisture and depth to the water table. Parameter values for this parameterization are listed for 11 different, widely applied soil texture descriptions. The high numerical efficiency of the proposed method makes it suitable for inclusion into demanding applications, e.g., a Monte Carlo framework, or high spatial resolution.