Prediction of Stream Discharge and Sediment Runoff Accompanied by Rainfall in Mountainous Areas

The management of the forest in Japan is changing because of declining of the forestry. As a result, forests are not able to get much sunshine, decrease undergrowth, and become the bare land. Then, the basins which have deteriorated forest after the aforementioned process are in danger of slope failures when it rains heavily. Thus the state of sediment runoff is influenced by the condition of the forest.The purpose of this study is to clarify the relationship among rainfall, surface water and sediment volume. The studied basin is the Nishitani River, a branch of the Tonda River, in Wakayama Prefecture, Japan, where is covered by coniferous forest. And, we conducted both observational and computational studies there. About the observation, we have measured precipitation and water level by a rain gauge and pressure-type level gauges, and sediment volume by a pit at the end of the basin. About the calculation, the surface water is estimated by a two-stage tank model and bed load volume is estimated by Ashida-Michiue formula. In particular, the reason why the two-stage tank model is adopted here is because it can treat runoff separately into 3 parts, the surface water, the subsurface water, and the groundwater. The simulation models were applied to several rainfall events in 2014. Predicted hydrographs by the two-stage tank model reproduce the events appropriately within the error values of 3.9%. On the other hand, the orders of the sediment volume due to typhoons 1411 Halong and 1412 Nakri matched between the observed and the estimated values, although the estimated value itself was 4 times larger than the observed one. In addition, the same parameters of the two-stage tank model were applied to the other rainfall events in 2015 and 2016. However, the repeatability was not so good. The reason can be attributed to flood caused by typhoon 1511 Nangka, bringing the change of some parameters. Furthermore, to know the effects of different species of the forest on stream discharge and sediment runoff, the calculation will be applied to another basin, the Kumanodani River nearby the Nishitani River, where in contrast River broad-leaved forest covers.