Author(s)

Koji Tamai

Abstract

Forests have the functions of ‘flood control’ and ‘water resources conservation’. The forest manage- ment system for suitable water runoff will change according to the relationship between two functions whether trade-off or compatible. Therefore, the relationship between the two functions is examined in this report. The simulated water runoff from watersheds those were once bare hill and were covered by forests with 0 to 100 years old were analysed. The process-based model that reproduces each sim- ple process of water movement in the forested basin was used to simulate the water runoff under the conditions of different forest soils and vegetation. As results, the developed forest soil has the effect to reduce runoff during high water and increase runoff during drought. This effect is called ‘runoff delay effect’. On the other, developed forest vegetation has the effect to reduce runoff during low and drought due to evapotranspiration, but has little effect on runoff during high water. These results mean that the both forest functions of ‘flood control’ and ‘water resources conservation’ derives from the work of the developed forest soil. Then two functions are concluded to be compatible. From the above results, forest management system for suitable water runoff should have the first purpose of forest soil conservation. Forest vegetation plays an important role in conserving forest soils. Poor forest vegetation results in sur- face erosion and land slide resulting in loss of forest soil. Even when harvesting forest vegetation, the works should be taken to preserve the forest soil. In areas where water resources are scarce, suboptimal measures may be used to reduce forest vegetation to reduce evapotranspiration. It may be necessary to determine the amount of forest vegetation by comparing the risk of soil loss and the expected increase in drought runoff.

Keywords

forest soil, forest vegetation, flow duration curve.