The Studies on the Runoff from Soil Surface (1)

Effect of initial moisture content on surface runoff

Abstract

The flow through soil after precipitation, the part of which appears as surface runoff, can be divided into following three stages: 1) free infiltration observed before surface storage occurs; 2) infiltration during the advance of wet front through soil after the surface storage; 3) consequent steady state flow.
Surface runoff is closely related to initial moisture content in stages 1) and 2).
The present paper is an attempt to evaluate the effects of initial moisture content on surface runoff.
The time between precipitation and surface runoff, as well as infiltration rate, decreases as the initial moisture content increases. It follows that the causes of the decrease are the presence of entrapped air and the downward movement of the initial moisture in soil.
The auther derived the rate of surface runoff, which is a function of time, from the infiltrationtheory, and obtained the following formula:
Q= (r-K) (1-t_s/t)...(1)
where Q is the rate of surface runoff, r is rainfall intensity, K is Darcy's permeability, t is the time after beginning of rainfall, and t_s is the time between beginning of rainfall and that of surface runoff. Formula (1) agreed with the experimental data.