Effects of agricultural land consolidation on erosion processes in semi-mountainous paddy fields of Japan

In semi-mountainous paddy fields of Japan, severe erosion occurs as a result of heavy rainfall or storm. In view of the considerable significance of agricultural land conservation for sustainable agriculture in semi-mountainous paddy fields, this study was conducted to show erosion phenomena and to consider the conservation effects of agricultural land consolidation which is carried out to make field sizes larger, mechanization easier and productivity per unit area higher. As the paddy field surface is flooded, levelling of adjacent surfaces and construction of levee slopes are essential. Also, soils of subsurface layers in paddy fields and levees are compacted to decrease percolation. Erosion in paddy fields occurs mostly on levee slopes, so soil structures in levee slopes were examined to explain erosion processes. The number of erosion events per hectare in consolidated paddy fields was smaller than that in non-consolidated paddy fields. Additionally, soil permeabilty at the toe of non-consolidated levee slopes was lower than that at the top. It was judged that soil structural changes were brought about by percolating water in non-consolidated levee slope. During heavy rainfall, water percolates through levee slopes and increases soil water pressure and degree of saturation in the upper zone of clay deposition. Consequently, these conditions cause erosion phenomena such as slope failures or gully erosions. Therefore, agricultural land consolidation appears to be of importance in modifying soil structural changes and in reducing erosion events. (C) 1996 Silsoe Research Institute