2018 Volume 31 Issue 5 Pages 399-413
To observe the temporal changes of the surface runoff and sediment yields in forest slopes under conditions of high rainfall intensity, with consideration of the changing kinetic energy of a raindrop in the process of rainfall passing through the canopy, a new rainfall simulator was designed to sprinkle water above the canopy. Usefulness of the simulator was evaluated on a Japanese cypress (Chamaecyparis obtusa) plantation located in Gunma Prefecture, Japan. Intermittent water sprinklings was performed with durations of about 13, 32, and 16 min. Surface runoff and sediment yields were monitored using a bounded monitoring plot. The throughfall amount, raindrop sizes, and drop velocity were measured over the plot using a laser precipitation monitor. The throughfall intensities were between 195-378 mm h-1, which were judged to be reasonable based on the water balance in surface soil. Results of comparing the drop size and the drop velocity in a forest and in an open field indicated that the ratio of raindrops with larger size increased during passage through the canopy. Both the drop velocity in the forest and in the open field were lower than the terminal velocity and the velocity calculated using the branch height of the canopy. A positive correlation (r=0.96) was found between the throughfall intensities and the infiltration rate. As the rainfall continued, the surface runoff ratio did not change much. However, the ratio of sediment yield to kinetic energy of throughfall and surface runoff tended to decrease.
