Abstract
In Northeast Thailand, permeable sandy soils are widely distributed. Most rainfall may infiltrate the ground and there may be little surface runoff. However, soil erosion caused by surface runoff has been reported. To investigate the mechanism by which surface runoff occurs, we selected a sloping area where the top soil consisted of a permeable sandy layer overlying a very low permeability clay layer. We defined the soil water storage capacity of the sandy layer at a particular time as the difference between the maximum observed water content and the observed water content at that time. We assumed that when the amount of rainfall exceeded the soil water storage capacity, the excess rainfall became surface runoff. As a result of monitoring of soil water content in the sandy soil layer over a period of one year, the total amount of runoff was estimated to be about 30 % of the total rainfall and the periods for which our analysis showed that soil water storage capacity was exceeded were followed by rises in the water level of a pond that surface runoff flowed into.
