Influence of Vegetation and Management Conditions of Forest Cathcments on Their Runoff Characteristics

There is rising concern that devastation of planted conifer forests due to poor management might change runoff characteristics and provoke severer flood disasters in Japan. Planted conifer forests in Japan were planted from 1960s to 1970s as a countermeasure to building materials shortage. Domestic forestry products, however, lost out in the price competition with imported products after 1970s, so that forestry has stagnated in Japan, which causes the shortage of forestry workers to maintain planted forests. In this study, runoff variations for the same rainfall pattern observed at neighboring catchments with different conditions were compared so that influence of vegetation and management conditions of forest cathcments on their runoff characteristics was examined. Besides, a Long-and-Short Term (LST) model was calibrated for each of the objective catchments to know the difference in hydrological components as surface flow, subsurface flow, groundwater flow, infiltration and percolation. In three observation sites, eleven catchments were selected as objectives. The areas of the catchments vary from 0.17 to 36 ha. First, runoff characteristics between conifer and broadleaf forests was compared. The result clearly showed that total runoff depth and peak runoff for storm events were both larger in conifer forests than in the broadleaf catchments, and that total infiltration was smaller in conifer than in broadleaf catchments. The result supports the fact that natural forests are generally considered to have functions of water retention and flood control. Second, runoff characteristics between well and poorly managed planted conifer forests was compared. The result, however, did not show the clear difference. It was difficult to find correlation between forest management condition of catchments and total amount of each hydrological process during some storm events, so that the existence of forest management impact on runoff characteristics could not be identified only by this investigation. It is probably due to mutual interaction of the forest management effect with the other factors as topography, catchment area, soil characteristics, strongly affects runoff characteristics. These results suggest that more investigation is required.