Setting priorities for floods mitigation through forest restoration: The threshold elevation hypothesis

Nature based solutions (NbS) for flood regulation (e.g., forest restoration) need to be informed by the analysis of climate change and land-use/cover change (LUCC) effects on floods, but these effects are still poorly understood. In this study, it is hypothesized that effects of climate change and LUCC on floods exhibit an abrupt change at a threshold elevation with implications for forest restoration. The study was carried out in the Tena watershed located in Ecuadorian Amazon. Hydrological simulations were run using TETIS model for different climate and LUCC scenarios. Projected precipitation from the Global Climate Models (GCMs) under the SSP5-8.5 scenario of CMIP6 was assessed. Isolated and combined effects of climate change and LUCC on floods across an altitudinal gradient were analyzed at 42 flow sites. Obtained results confirm the hypothesis showing the existence of a threshold elevation at 590 m a.s.l., where abrupt changes on floods occurred. The effects of LUCC prevailed over the effects of climate change in the upper basin, while in the lower basin, effects of climate change prevailed, especially for small and medium flood events. The results suggest that native forest is priority in the area above the threshold elevation, informing restoration as a NbS for flood regulation in humid tropical basins in a context of climate change.