Nature-based solutions for floods AND droughts AND biodiversity: Do we have sufficient proof of their functioning?

Abstract Climate change and human-modified landscapes have led to an increase in global flood and drought risks, while biodiversity has declined. The concept of using nature-based solutions (NbS) to improve the water retention capacity at the landscape scale, also known as ‘sponge functioning of catchments,’ has been recognised to help reduce and delay peak flows and stimulate infiltration to the groundwater, thus reducing flood and drought risks. Although various effects of NbS have been demonstrated, there is limited evaluation of the combined multiple benefits for flood risk reduction, drought risk reduction, and biodiversity. To address this gap, we analysed various online databases on NbS and additional literature on the evaluated combined effects of NbS. We found that the quantitative evaluation of NbS is fragmented and not standard practice in many projects. Although many successfully implemented NbS have been reported in different environments globally, most cases lack evidence for their response to the combined impacts of floods, droughts, and biodiversity. Therefore, we propose four components to facilitate planning, design, implementation, and monitoring of NbS that improve sponge functioning for floods and droughts. First, we suggest increased understanding of how NbS affects the hydrological processes of both flood and drought events along the full range of potential conditions. Second, we recommend evaluating the effect of potential NbS measures at a landscape scale. Third, we propose that integrated modelling and upscaling techniques should be improved to quantify the impacts of NbS. Finally, we suggest using a consistent and socially relevant set of indicators to evaluate the NbS and communicate this with stakeholders. In conclusion, our analysis demonstrates a need for more comprehensive and standardised evaluation of NbS, particularly in relation to their combined impacts on floods, droughts, and biodiversity.