Abstract
To clarify the effect of forest thinning on evapotranspiration, thinning was conducted within a basin (HV, 0.88 ha) with planted hinoki (Chamaecyparis obtusa) and sugi (Cryptomeria japonica). This basin lies within the Hitachi Ohta Experimental Watershed, in the northern part of Ibaraki Prefecture, Japan. Thinning was 50% of trees and 30% of timber volume. Daily evapotranspiration for HV and calibration basin HA (0.84 ha, same vegetation) was estimated using the short-period water budget method. The regression curve between HV and HA for daily evapotranspiration before thinning was determined. Daily evapotranspiration in HV when thinning was assumed to have not been done was estimated from the regression curve based on daily evaporation in HA. The change of evapotranspiration after the thinning was shown by the difference between the actual measurement value and estimated value in HV, using the paired catchment experiment. Evapotranspiration estimated by atmospheric conditions was estimated by the Priestley-Taylor equation and used as a rough standard of daily evapotranspiration. Our results show that yearly evapotranspiration reduced about 17% after thinning. The effect of thinning on evapotranspiration appeared gradually after six months, and was more pronounced after two years. Additionally, the effect was strong from June to October.
