Rain Infiltration into swelling/shrinking/cracking soils

A model for predicting rain infiltration at the field scale with swelling/shrinking/cracking/soils is described. The simplifying assumptions are: (1) no vertical infiltration takes place due to a sealed surface condition. rainwater moves laterally over the soil surface to the cracks, where it uniformly flows along the vertical walls of the cracks. (2) The geometry is represented by a prismatic column structure with cracks between the columns. The approach consists of a two-component process of Darcian matrix flow in the soil medium and Hortonian flow on the walls of the cracks. The Darcian analysis of horizontal matrix flow uses a spectral series solution of the Richards equation for the wetting front advance. The crack volume is determined from bulk density measurements, from which ponding time estimates can be made. Closed-form expressions are derived for the cumulative infiltration. The analytical results are compared with experimental results obtained for a Mississippi Delta clay soil. For the case, where excess rainwater has reached the bottom of the crack at the moment crack closure occurs, an exact solution is obtained for incipient ponding as a function of crack morphology, rainfall intensity, and sorptivity. Runoff is computed as the difference between the cumulative rainfall and cumulative infiltration.