Abstract
The forest watersheds are located in the upper reaches of the basins and water source area draining to the Ariake-Yatsusiro Seas, and the forest area functions as a rain fall catchment area. Rainwater, sediment and nutrient flow out of the forest area into the middle basin and then to the bottom basin. The water source area has a very local characteristic and also the nature of the rainfall-runoff varies over a wide range. Data taken from each source area all the way to the bottom basin are necessary for an evaluation of the influence of the forest area. However, there are few such observational data available, although data of observational values of “the amount per unit area” are necessary for the individual basins. Thus general runoff characteristics from the forest area cannot be explained. In this paper, first, the general water balance in the forest area was discussed, and second, sediment discharge from the forest area was shown to be within the range of normal geological erosion. The runoff of nutrient from the forest area was lower by one order than nutrient (Total-Nitrogen (T-N) and Total-Phosphorus (T-P)) flowing into the Ariake-Yatsusiro Seas. But, the T-N from the abandoned Hinoki (Chamaecyparis Obtusa Endl.) forest was estimated to be the same order of T-N flowing into the Ariake-Yatsusiro Seas. Accurate measurement of the nutrient during rainfall and flooding, from the upper to the down reaches of rivers, are not presently being undertaken. For this reason, an accu rate assessment of the runoff of rainwater, sediment and nutrient from the forest area isn't clear. It is noted that the conservation of the soil layer is the most important for forest management.
