Elucidation of causal chains and restoration applying agricultural engineering approaches for watershed degradation to the Lam Phachi River, Thailand

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We conducted a case study in the Lam Phachi River Basin, Thailand in order to clarify causal chains for watershed degradation using many types of field survey techniques, and satellite image analysis etc. Main results are as follows.1) The Lam Phachi River Basin was almost covered by forests until the beginning of the 1970's, however they were not natural forests but secondary forests due to the sustainable shifting cultivation conducted by mountain tribes. The forests has decreased to 50-60 percent in area until 1990 because of widespread conversion of the forests to cultivated lands caused by settlers since the 1970's. Recently pineapple fields are expanding to the sloping area of the target area of the basin due to a shift to a more profitable cash-crop culture since the 1990's.2) The forestlands had higher final infiltration rates (44-160 mm/hr) than the pineapple and cassava fields (5-39 mm/hr). Such low infiltration capacities of cultivated lands and strong rainfall intensities of tropical region caused severe soil erosion (69, 163 t/ha/yr) in some pineapple fields on the sloping areas. Thus the conversion to cultivated lands during the last three decades, particularly the expansion of pineapple fields to the sloping areas during the last decade, increased the susceptibility to accelerated erosion.3) Through this study, we understood the vicious cycle of watershed degradation as follows. If the critical point in deterioration is exceeded by inappropriate agricultural practices, accelerated erosion becomes self-sustaining, that is, the more surface runoff occurs, the more soil is removed and the less water and nutrients remain to support the plant growth, therefore the site cannot recover natural processes.4) Dynamic changes in cultivation during the last decade might have already caused the vicious cycle of watershed degradation in some areas of the basin (e.g., pineapple fields on the sloping areas).