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

为了明确流域退化的因果链，我们在泰国的Lam Phachi河流域进行了案例研究，使用多种类型的实地调查技术和卫星图像分析等。主要结果如下：(1)直到20世纪70年代初，蓝帕奇河流域几乎被森林覆盖，但由于山地部落持续的轮番耕作，这些森林不是天然林，而是次生林。到1990年，森林的面积已经减少到50- 60%，因为自20世纪70年代以来，定居者广泛地将森林转化为耕地。最近，由于自20世纪90年代以来转向更有利可图的经济作物文化，菠萝田正在扩大到盆地目标区域的坡地。2)林地的最终入渗速率（44 ~ 160 mm/hr）高于菠萝地和木薯地（5 ~ 39 mm/hr）。由于耕地入渗能力低，热带地区降雨强度强，导致坡耕地部分凤梨田土壤侵蚀严重（69,163 t/ha/yr）。因此，在过去的三十年里，耕地的转换，特别是在过去的十年里，菠萝田向坡地的扩张，增加了对加速侵蚀的易感性。3)通过本研究，我们了解了流域退化的恶性循环。如果不适当的农业做法超过了恶化的临界点，加速侵蚀就会自我维持，也就是说，地表径流越多，土壤流失越多，维持植物生长的水分和养分就越少，因此场地无法恢复自然过程。4)在过去十年中，种植的动态变化可能已经在流域的某些地区造成了流域退化的恶性循环（例如坡地的菠萝田）。

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).