LONG TERM VARIABLE PROPERTIES OF RUNOFF PROCESS IN A MOUNTAINOUS FORESTED CATCHMENT

摘要

This paper investigates the changes of fundamental hydrological properties with paying attention to climate change and forest growth change after thinning by using long term (24 years) data records at Futatsumori research area in Nakatsugawa City, Japan. The discharge ratio estimated as the ratio between streamflow and rainfall volume over a certain area and time, as a function of the catchment state conditions. Some seasonal cyclic variations or long term trends are derived from the hydrological data. The long term discharge ratio in all analysis cases exhibits a decreasing trend and it is assumed to be caused from increases of temperature, evapotranspiration and rainfall interception with growth of forest. Comparison of discharge ratio between the period of 1984 to 1989 and 2002 to 2007 have resulted for 100 and 200 mm rainfall event are11% and 14%, respectively. The highest difference of discharge ratio was observed in summer season (Jun-Aug) pointing at 15% and 19%, respectively. Therefore the results of decreasing recent discharge ratio imply that the dealing with discharge ratio coefficient as constant value is not appropriate. In other word the result of response against both growth of forest and temperature rising also show water resource may decrease in future, if rainfall situation was same.