2008 Volume 21 Issue 2 Pages 126-139
The behavior of rain water that reached the forest floor during a large scale rainfall event was clarified on a mid-lower position of slope in a natural forest, western Kochi, an area known for heavy rainfall, in order to discuss the influence of such water movement on nutrient dynamics or occurrence of the landcollapse. Throughfall, the overland flow, the soil water storage of 0 to 50 cm in depth, and the output of a tension-free lysimeter (TFL runoff) at 50 cm in depth were observed during two events with a total rainfall 642 mm (2005/9/4-7) and 355 mm (2006/8/16-19). Near the research site, the precipitation and the catchment runoff were also observed. We found that the runoff percentage of the overland flow to the throughfall was low, and the upper limit (about 60 mm) for an increase of the soil water storage during an event(ΔS) was determined. After ΔS exceeded about 40 mm, the TFL runoff demonstrated that about 70 % of throughfall drained to soil layer deeper than 50 cm at almost same intensity and time (delay within 30 minutes) as throughfall. The peaks of catchment runoff occurred at almost the same time as those of throughfall and the TFL runoff. Therefore, the subsurface stormflow, not the overland flow or the soil water storage, was important for runoff processes in large scale events, suggesting that the subsurface stormflow contributes to the formation of the direct runoff. The above indicates possibilities that the fast subsurface stormflow prevents the landcollapse and runoff to stream under little reaction to soil or bedrock during an event.
