Evaluation Index of Flood Reduction Function of Forest Watershed Focused on Surface Runoff Discharge

It is thought that forests have the flood reduction function that decreases flood peak discharge and delays flood peak time. In this study, a flood reduction evaluation index was suggested on that aspect. Using this index, relations between the flood reduction function of forest and forest parameters are able to be discussed.

For the evaluation of the function, it is common to use amount of rainwater storage under the ground, and ratio of runoff discharge for total rainfall. However, in this study, it was thought that evaluation of surface runoff component discharge was important, because it is main component of flood peak discharge.

Therefore, the evaluation index of flood reduction function of forest was built using two parameters. They are the storage capacity of forest soil for rain water and the ground surface rough degree to delay surface runoff flow. The index was applied to three forests that varied in forest parameter. The usefulness of the index and the relations between the flood reduction function and forest parameters, such as soil layer thickness and ground surface rough degree, were discussed.

Two steps tank model was used for calculating storage capacity of forest soil and ground surface roughness. By the mode, flood flow discharge is isolated to groundwater runoff component, sub-surface runoff component, and surface runoff component. Slope incline, outer soil layer thickness, and ground surface rough degree are incorporated in parameters for surface flow discharge calculation. Therefore, the index is helpful to evaluate forest properties contributing to the reduction of flood peak flow discharge.

In this study, three forest basins were studied. They are natural beech forest, artificial coniferous trees forest and needle wide mixture forest. At first, three tank models were built. Each tank model has soil thickness parameter and the ground surface rough degree parameter that can reproduce the rainwater runoff mechanism of each basin. Then, all models were given a same rain wave pattern, and storage and discharge were estimated, and the index of each forested basin was estimated. The main results are as follows. 1)　The flood reduction evaluation index is useful. 2)　The clear relations were not accepted between the flood reduction function and the forest type. 3)　Ground surface rough degree contributed to reduce the surface runoff and delay the flood peak time.