After the Horton overland flow concept became legend until the early 1960s, new findings then revised this concept for implementation in several catchment area conditions. Runoff processes in tropical regions seem to have specific characteristics such as thick humus on the top layer of the catchment area and non-uniformity of hydraulic conductivity vertically and horizontally across the stream. To overcome the runoff problems in tropical regions, a reasonable model needed to be developed to predict generated runoff. Based on the concept that saturation condition in a catchment generates non-uniformly during the course of a rainfall, and that the storage capability of top layer in intercepting rain-water leads to subsurface and return flow, the water balance in a segment of the catchment area is calculated. The mechanism of flow in the model is that subsurface flow and return flow, which are a function of hydraulic conductivity, and surface runoff on a segment are integrated for the whole areas. The generation of flow using the model seems to indicate that runoff peaks will commence after each segment is saturated starting from the valley bottom. The lag time for initial flow to alter significantly depends on the data on stream-flow before the storm began that indicate the soil moisture and depth of the temporary water table. Generally, though, the model is very flexible, so that data on each segment subject to the wide variety of physical conditions of the catchment area characteristic of tropical regions can easily be implemented.

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