Exploring Nature-based Solutions in Mountain Regions: A Review of Their Hydrological Functions

Abstract. Mountain regions play a fundamental role in sustaining the water supply for many activities around the world. However, these ecosystems are under increasing pressure due to climate change, changes in land use, and socioeconomic stressors, threatening water security. Implementing Nature-based Solutions (NbS) in mountain catchments are considered and effective means to increase water security, in the mountain regions itself, but also downstream. Despite growing interest, the scientific evidence base on the hydrological impacts of NbS in mountain regions is limited and characterised by methodological inconsistencies and insufficient consideration of climate dynamics. This study aims to review the hydrological performance of NbS in mountain regions and identify key knowledge gaps. We present a typology covering Wetlands, Forest-based interventions (Afforestation, Reforestation, Forest Conservation, and Agroforestry), and Water Harvesting interventions highlighting their hydrological functions. The analysis revealed that the most frequently reported indicators included flow regulation, groundwater recharge, water retention capacity, and soil erosion control. Our findings emphasise the need for more standardised methodologies in this environment, to enable robust comparisons of NbS performance under future scenarios. Strengthening the evidence base will require methodological integration and multiscale analyses that incorporate climate variables, as well as the inclusion of local and indigenous knowledge in planning and evaluation processes. Such efforts are essential for enhancing the design, implementation, and long-term effectiveness of NbS strategies for safeguarding water resources in mountain regions.