Surface runoff (or overland flow), which is generated by the precipitation that falls within a drainage area (catchment, watershed), is governed by several factors and processes, including rainfall rate and duration, the characteristics of infiltration (capillary imbibition and gravity-driven) and the temperature regime of soil, landscape surface characteristics, vegetation type, and some others. Hillslopes are regarded as a basic element of catchments, therefore the mathematical and physical description of the hydrological processes that occur at the hillslope scale is the first step to designing more general hydrological models describing hydrological response at catchment/watershed scale.