Author : Naree Suddhapreda

Summary : Experiments were conducted to study the effects of rainfall intensity, slope steepness, and crop cover on runoff and soil loss. The physical model of estimating soil erosion as developed by Rose, Barry, and Sander (1983) was evaluated, and the simplified equation was used in predicting soil erosion of two watersheds (Haui Pong and Nam Poi) in Chiang Mai, Thailand. Treatments involved two soil types (Korat sandy loam and Pak Chong clay loam, both belonging to Oxic Paleustults), three levels of rainfall intensity (60, 90, and 120 mm/hr), four levels of slope steepness (0, 5, 10, and 20 percent), and two kinds of cover crops (corn and soybean). Rainfall intensities were controlled using a rainfall simulator and slope steepness using a slope-adjstable soil trough. Runoff, sediment concentration, and soil loss increased with increase in rainfall intensity, and slope. Sediment concentration was high at the earlier stage of crop growth, but this exponentially decreased as the vegetative cover increased with time. The sediment concentration in Pak Chong clay loam was generally lesser than in Korat sandy loam, particularly under higher rainfall, steeper slope, and lesser vegetative cover, indicating that the latte soil is more erodible than the former. The physical model yielded good prediction of soil erosion. In both soils, under bare or cropped soil surface conditions, the correlations of the predicted and the measured soil loss on different combinations of slope and rainfall intensity were highly significant. In the two watersheds, the predicted soil erosion values in the forest, and the well-vegetated and level agricultural lands were less than 4 tons/ha/yr. On the other hand, the hilly areas utilized for shifting cultivation gave the highest value (1208 tons/ha/yr).