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

在泰国北方和东北部的坡地农田上，设置了3个试验小区，在季风气候条件下，利用水动力监测系统，定量研究了径流的产生条件和过程。在小尺度（1 m × 1 m或25 m × 2.5m）田间小区内，每隔10 min用数据记录仪连续监测降雨量、土壤体积含水量和地表径流量。然后将水源性固定降雨的去向分为土壤水分增量、地表径流和水分流失三个去向。在此基础上，确定了降雨强度、土壤初始含水量等因素对土壤水分流失率和地表径流量的影响及其产生的初始条件。结果表明：1）土壤水分流失率和地表径流量均与降雨强度密切相关; 2）雨季初期，降水主要通过蒸散作用散失，部分以毛细水的形式积累到土壤中：结果表明，土壤水分在整个雨季逐渐增加，特别是在中、细质地土壤中; 3）雨季后期，土壤内部排水流失量增加; 4）雨季降雨强度和土壤水分状况在各试验小区间的分布规律不同，导致以下类型的产流风险：i）泰国东北部低海拔地区的降雨强度依赖风险; ii）泰国北方中等海拔地区的毛管饱和增强风险; iii）泰国北方高原雨季后期降雨强度增加风险

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