The Relationship between Large Wood Export and the Long-Term Large Wood Budget on an Annual Scale in Japan, Using Storage Function with the Lumped Hydrological Method

In this study, we aimed to verify the two relationships on large wood export, as follows: (1) the relationship between large wood recruitment and landslides triggered by intense rainfalls and (2) the relationship between large wood export and the long-term large wood budget on an annual scale, based on the direct export of large wood caused by an increase in large wood recruitment with extreme rainfall events, as well as the baseflow of large wood, which is mainly old large wood recruitment stored at the slopes and in the stream. To reproduce these two relationships, the model consisted of two frameworks, as follows: (1) the rainfall-induced analytical shallow landslide model, with 30 m spatial resolution for large wood recruitment and (2) the double/triple storage function, with the lumped hydrological method at a watershed scale for large wood entrainment. Application of the model to 212 dam reservoir watersheds across Japan resulted in reproducibility in the estimation of large wood export volumes in 134 of the target dam reservoir watersheds, which contribute 63.2% of the target basins. This indicated that our results verified these two relationships as primary relationships. To analyse the difference in large wood export systems, a frequency analysis was conducted using correlation analysis based on large wood export volume and the cumulative values of six patterns of large wood recruitment volumes. The results indicated that there might be differences in large wood export systems between the watersheds represented by the double storage function model and those represented by the triple storage function model.