Average landslide erosion rate at the watershed scale in southern Taiwan estimated from magnitude and frequency of rainfall

This study calculated the long-term average landslide erosion rate in the Kaoping River watershed in southern Taiwan and investigated the relative importance of extreme rainfall events on landslide erosion. The method followed three steps: first, calculating landslide volumes for 10 rainfall events from a multi-temporal, event-based landslide inventory; second, estimating the frequency of landslide-generating rainfall by using hydrologic frequency analyses; and third, combining the two sets of data to estimate the average landslide erosion rate. Results of the study showed that the average landslide erosion rate is 2.65-5.17mmyr^-1, corresponding well to rates reported in other studies using other methods. The study also found that extreme-intensive rainfall events play a more important role on landslide erosion than frequent-moderate rainfall events. Extreme rainfall (maximum 24-h rainfall >600mm) contributes 64-79% of the average landslide erosion rate. Moreover, the natural variation of landslide erosion magnitudes is extremely large and can cause significant uncertainty in estimating the landslide erosion rate from total landslide volume. This study found ±1.2mmyr^-1 of uncertainty based on simulation results involving a hypothetical 100-year landslide inventory. In summary, this study demonstrates the importance of extreme rainfall events on landslide erosion, and the method proposed in this study is capable of calculating a reliable estimate of average landslide erosion rate in areas with insufficient landslide records.