A COUPLED SOURCE-RESPONSE/RICHARDS-EQUATION MODEL FOR SIMULATING INFILTRATION IN THE PRESENCE OF PREFERENTIAL FLOW PATHS

Preferential flow paths, such as macropores, root channels, and worm holes exert a marked influence on infiltration and subsequent subsurface redistribution of water applied on land surface. Traditional models of subsurface flow, which assume only diffuse (surface-tension viscous) flow and are based on the diffusion equation or the Richards equation, are often incapable of accurately simulating these phenomena. Over the last few decades, a number of approaches have been suggested for simulating water movement through soils with preferential flow paths; these include kinematic wave, transfer function, and water-content wave models among others. The recently proposed source-response model (Nimmo, 2010) is unique among such models in that water transfer from land surface to depth is controlled by the water-application rate at land surface.

This presentation describes a coupling of the source-response model with a one-dimensional version of the Richards-equation based model of variably saturated flow, VS2DT. The new coupled model can simulate flow within the preferential (source-response) domain alone, within the diffuse (Richards-equation) domain alone, or within both domains simultaneously. Water exchange between the two domains is treated as a first-order diffusive process. The coefficient in the exchange equation can now be treated as a moisture-dependent function, as opposed to the constant value assumed in the original source-response model. Two sets of experiments were simulated: infiltration to a soil core with natural structure under laboratory conditions and infiltration to a grassland field site. Simulation results compare well with published data for both sets of experiments. The coupled model can be used to elucidate the roles of diffuse and preferential flow in infiltration and redistribution processes. Sensitivity analysis can identify key parameters affecting these processes, thereby shedding light on conditions that favor either preferential or diffuse flow.