An extended kinematic-wave theory for infiltration in soils with declining porosity causing delayed perching

Understanding the infiltration process in heterogeneous soils can help improve its representation in Land Surface Models. Here we consider one-dimensional (1D) infiltration into a low-textured soil with declining porosity and hydraulic conductivity with depth. The reduction in conductivity leads to the delayed formation of a perched water table which expands both upwards and downwards. We model this infiltration process by extending the kinematic wave approximation to include the fully saturated region. In 1D this is possible, because analytic expressions for the flow in the saturated region are available. We provide a general framework to solve gravity-dominated infiltration problem for different permeability models and porosity variations with depth. Solving this simplified problem is also computationally more efficient than solving the full Richards equation, because the non-linearity can be integrated explicitly. This approach is applicable in soils where infiltration is primarily driven by gravity and the capillary transition zone is small. We also investigate the consequences of neglecting capillarity during infiltration into a multi-layer soil through experimental data from Childs and Bybordi (1969) and model inter-comparison.

Figure shows the evolution of transitional infiltration into a two-layer soil with a porosity φ jump at depth z'=1. The soil does not saturate initially as the infiltration rate I is less than the infiltration capacity fc (see Figure b). However, perching occurs at time t's due to decline in hydraulic conductivity (see Figures c-d). The perched water table rises up to the surface leading to ponding at time t'p (see Figures d-f). This corresponds to a sharp reduction in infiltration rate as shown in Figure (g). The analytical solutions (A) show excellent agreement with numerical simulations (N) as well as Hydrus-1D results (H).