Quantification of the effect of forest harvesting versus climate on streamflow cycles and trends in an evergreen broadleaf catchment

A new method known as Unobserved Components-Dynamic Harmonic Regression (UC-DHR) was applied to a 39-year record of rainfall and streamflow for three sub-catchments of the Sarukawa Experimental Watershed in southwestern Japan. Some 25% of the timber was harvested from one of the sub-catchments in May-July 1982 and the objective was to quantify the magnitude of this effect relative to the effects of climate cycles (e.g. Southern Oscillation Index). The observed effects of inter-annual climate cycles (i.e. 0.89-1.36mm/d) were seen to be comparable (i.e. 0.70-1.17mm/d) to the effects of harvesting 25% of the standing timber. This result underlines the importance of always quantifying the effect of climate on streamflow response when harvesting impacts are studied.