ABSTRACT
The drainage basin provides the natural framework for the generation of river flow. As such, it is widely recognized as the natural unit for water management and for the scientific study of hydrological response. The infiltration of water into the soil is perhaps the most pivotal process within the drainage basin. According to Horton's theory, the hydrologically effective rainfall is all the water that remains on the surface after infiltration and any losses in surface detention, interception and evaporation. Both surface and shallow subsurface sources are now known to consist of a range of processes from infiltration-excess overland flow, saturation overland flow and rill flow to saturated and unsaturated diffuse through flow, macropore flow and pipe flow. This return flow occurs over only part of the catchments, typically on lower slopes where soil water seepage has developed higher soil moisture levels prior to the storm and where groundwater levels may be nearest the surface.
