Abstract
Paddies in hilly rural areas are supposed to have beneficial effects on the environment and many valuable functions, such as flood prevention and the fostering of water resources etc., when it is appropriately used and managed by farmers. However, farm lands in adverse conditions of water and terrain in rural areas have been abandoned due to the aging of farmers. No research has been done on the effects of the abandoned farm lands on runoff except for some assumptive results estimated using a model. In this paper, the change of runoff characteristics due to the abandonment of rice cultivation in hilly areas was examined, based on the observed runoff and soil sample data from both cultivated and abandoned paddy fields. In addition, the relation between soil and water and runoff properties was analyzed in connection with unusual weather conditions such as a long rain spell in the summer of 1993 and a severe drought in 1994. The results obtained are summarized as follows:
1) It was shown that the successive change of runoff mechanism tended to occur with a paddy field abandoned in a hilly rural area of the Hokuriku region. That is, the runoff ratio and runoff peaks remained increased for 5 years after its abandonment in these areas and thus paddies there had had the function of flood mitigation during that period. However, the tendency of increased runoff was completely reversed after the occurrence of the abnormal drought, i. e. the amount of runoff from the abandoned paddies was remarkably reduced after it.
2) The above-mentioned change in runoff in the abandoned plot was explained by the existence of flow routes on the surface before the drought and an increase in the number of cracks and macropores in the soil after the drought.
3) It was shown that the abandoned paddies are affected by a long spell of rainy weather in summer and abnormal drought, but their influence on the cultivated paddies is minimized because of maintenance and rehabilitation by farmers such as irrigation, puddling, the repair of soil cracks and so on.
4) Surface runoff from sloped levees is prone to occur rather quicker than that from planting lots, and the amount of runoff tends to be bigger due to the steep slope in hilly areas. These characteristics have to be considered in handling the runoff from farm lands in hilly rural areas because the area of sloped levees in those areas is not negligible.
5) One of the characteristics in the abandoned paddies is that the water level and soil moisture under the ground surface vary from place to place, such as mountains or valleys on the sides of the plot. These differences are factors which need to be examined in dealing with the runoff from paddies in hilly areas.
