Determination of ponding condition and infiltration into layered soils under unsteady rainfall

Few infiltration models, based on the Green–Ampt approach, in current use are suitable for simulating infiltration into heterogeneous soils of variable initial moisture distributions during unsteady rainfall. An algorithm is proposed for determining the ponding condition, simulating infiltration into a layered soil profile of arbitrary initial water distributions under unsteady rainfall, and partitioning the rainfall input into infiltration and surface runoff. Two distinct periods, pre-ponding and post-ponding, are taken into account. The model tracks the movement of the wetting front along the soil profile, checks the ponding status, and in particular, handles the shift between ponding and non-ponding conditions. The modeling also covers a fully saturated flow condition when the wetting front reaches the bottom of the soil profile (water table). Detailed procedures that deal with the variability in both soil properties and rainfall intensity are presented. For the purpose of model testing, three different cases (homogeneous soil and unsteady rainfall; layered soils and steady rainfall; and layered soils and unsteady rainfall) were discussed. Comparisons of the developed model with other infiltration models (both modified Green–Ampt infiltration model and fully numerical model) and field measurements were conducted and good agreements were achieved.