Study on initial process of soil erosion, on sloped, cropland in humid tropics using soil water-monitoring system

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The condition and process of runoff generation were studied quantitatively by a water dynamics-monitoring system Three experimental plots were installed on sloped agricultural field in northern or northeastern Thailand under monsoon climate. The amount of rainfall, volumetric soil moisture content, and the amount of surface runoff were continuously monitored for every 10 minutes using a datalogger, which was installed in a small-scale field plot (1m × 1m or 25m × 2.5 m). Then the fate of water derived firm rainfall was divided into three destinations, i.e. increment of soil moisture, surface runoff and water loss. Based on these data, the rate of water loss and surface runoff and the initial conditions for their generation were determined as functions of rainfall intensity, initial soil moisture content, etc. The results obtained are summarized below :1) Both the rate of water loss and surface runoff were strongly dependent on rainfall intensity ; and also on initial moisture content of the surface soil to some degree2) In early stage of rainy season, water delved firm rainfall is largely lost through evapotranspiration, and partially accumulated into the soil as capillary water : As a result, gradual increment of soil water throughout rainy season is observed especially in medium to the fine textured soils3) In late rainy season, a water loss through internal drainage increased4) Different distribution patterns for rainfall intensity and soil moisture condition during rainy season were observed among the experimental plots, resulting in following types of runoff generation risks :i) rainfall intensity-dependent risk in low elevation area in northeastem Thailandii) capillary saturation-enhanced risk in medium elevation area in northern Thailandiii) risk by increased rainfall intensity in later rainy season in highland in northern Thailand