Mountains play a crucial role in global water resources by generating disproportionally high runoff and delaying its release through snow and ice storage. This study quantifies the contribution of mountain runoff to lowland surface water withdrawal (LSWW) across all river basins larger than 10,000 km² worldwide, focusing on seasonal and interannual variability in the recent past. Additionally, we explore projected changes in lowland-mountain water interactions under climate and socio-economic scenarios.
Our results show that 15% of annual LSWW depend entirely on mountain runoff, with monthly variations ranging from 9% to 23%, highlighting strong seasonal reliance. An additional 51% of annual LSWW can originate from either mountains or lowlands. Under the SSP5-8.5 scenario, the absolute volume of lowland water withdrawal reliant on mountain runoff is expected to rise due to socioeconomic developments. At the same time, its relative share may decline on average in many basins as lowland precipitation increases. However, LSWW exhibits substantial interannual variability, with mountain runoff being most critical in years with low lowland runoff. While relative interannual runoff variability is lower in mountain regions than in lowlands in 70% of river basins, the absolute magnitude of runoff fluctuations is higher in mountain regions in nearly 70% of basins.
Our findings highlight the complex and evolving relationship between mountain runoff and lowland water use and reveal a large heterogeneity across river basins worldwide. Understanding these dynamics is essential for ensuring water security in both mountain and lowland regions as global environmental and socio-economic conditions change.