2009 Volume 22 Issue 5 Pages 342-355
To investigate the characteristics of direct runoff in a forested mountaious watershed, this study analyzed the relationship among precipitation, initial runoff, and direct runoff in the Fukuroyamasawa Experimental Watershed (35°12' N, 140°06' E, 2 ha) in University Forest in Chiba, The University of Tokyo, Japan. The watershed is located in a Tertiary formation and is covered by a mature mixed plantation of sugi (Cryptomeria japonica) and hinoki (Chamaecyparis obtusa) trees. Annual precipitation is 2170 mm, and annual mean temperature is 14.2 °C . In 1999, part of the watershed (1.1 ha) was clear-cut. Direct runoff increased as precipitation or initial runoff increased. Equations with three parameters were formulated to express the relationship among precipitation, initial runoff, and direct runoff. Direct runoff was accurately estimated by the equations(R2 > 0.94). The effects of precipitation and initial runoff on direct runoff were synergistic. The parameters of the equations changed after forest cutting. The effect of vegetation on direct runoff was found to depend on the size of the storm and was relatively high after medium-sized storms(about 100 mm). The relationship between initial runoff and direct runoff appeared to change in response to changes in the characteristics of moisture distribution in the watershed after clear-cutting.
