Long-term Variation of Relationship between Rainfall and Direct Runoff in Small Watershed with the Forest Restoration and Succession on Denuded Hills
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概要
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The objective of this case study was to quantify changes in the relationship between rainfall and direct runoff from the early period (1935∼1946) to the late period (2000∼2011) in the Ananomiya watershed, Ecohydrology Research Institute of The University of Tokyo Forests. The early period was defined as an initial stage of forest restoration on a denuded hill, whereas the late period was defined as a transition stage of forest succession and an initial stage of soil recovery. When total rainfall was 200, 300, and 400 mm, the estimated total direct runoff during the early period was 16.0, 25.8, and 33.5 mm, respectively, higher than that during late period. The difference in estimated direct runoff between the early and late periods was clear when the initial water condition was dry or the rainfall intensity was high. When the initial watershed condition was dry and when rainfall was 200, 300, and 400 mm, the estimated direct runoff during the early period was 19.1, 29.1, and 36.6 mm, respectively, higher than that during late period. When the rainfall intensity was high and when total rainfall was 200, 300, and 400 mm, the estimated direct runoff during the early period was 36.3, 56.7, and 71.3 mm, respectively, higher than that during the late period. However, when the soil condition was wet or the rainfall intensity was low, no difference was detected in direct runoff between the early and late periods.