An Estimation of Effective Rainfall on a Small Mountain Watershed

Abstract

A process to estimate effective rainfall in a small mountain watershed as a premise for estimating the flood hydrograph using the Unit-hydrograph method was studied.
The effective rainfall of each flood can be estimated through deducting infiltration losses from the rainfall in the watershed.
The method to estimated infiltration capacity is based on Horton's equation of infiltration capacity curve, where the initial infiltration capacity f⁰, final infiltration capacity f^c, and extinction coefficient k were analitically estimated according to the ydrological data.
As a result, f⁰ can be regarded as equal to rainfall intensity. The value of f^c revealed as constant. But varies over a long period by the recover of vegetations.
Simultaneously, it was found that the coefficient k varies with rainfall intensity and duration.
As the recovery in infiltration capacity during a drought is thought to be due to the depression of soil water content, the equitable value of the extinction coefficient β is estimated by a simulation method through setting up an equation between the soil water content and the infiltration capacity of the watershed.
According to the consideration of these results, an estimate process of the effective rainfall in time series was proposed.
By applying the proposed estimate process of hydrological analysis concerning the effective rainfall in the Ashio experimental basin, the process demonstrated reasonable results.