Dense Landslides Triggered by a Large Earthquake Reduced Evapotranspiration in a Hilly, Forested Catchment

Although many studies have investigated the effects of forest cover reduction, typically resulting from logging, on evapotranspiration or runoff in forested catchments, few have addressed the impact of landslides. In this study, we evaluated the effect of forest cover reduction caused by dense landslides on evapotranspiration in a hilly catchment that was originally entirely forested. The landslides, triggered by the 2018 Hokkaido Eastern Iburi Earthquake, led to the loss of 37% of the forest cover. Evapotranspiration, calculated using the short-term water budget method approximately 3 and 4 years after the earthquake, was 27% (15%–38% considering uncertainty) and 19% (5%–32%) lower than the evapotranspiration without the impact of landslides, as estimated using a forest evapotranspiration model based on the heat balance calculation developed in Japan. These reductions are comparable to those reported from logging activities. Furthermore, a comparison with a nearby site affected by windthrow suggested that landslides may exert a more severe impact on evapotranspiration, likely due to the reduction of both the forest canopy and the understory. Land managers should be aware that a reduction in evapotranspiration leads to increased runoff, which has the potential to heighten the risk of flooding and increase streamwater turbidity downstream.