Abstract
Snow droughts are increasingly recognized as an important feature of dry periods in mountain regions worldwide. While the phenomenology of this hazard is becoming clearer, its implications for hydrology, ecosystems, and upstream and downstream communities remain poorly understood. This knowledge gap leaves scientists and decision-makers without the necessary tools to support adaptation in the face of accelerating climate change and declining, increasingly ephemeral snow water resources. Leveraging 13 years of hydrological and multi-sectoral impact data from over 30 headwater catchments across Italy, we demonstrate how snow droughts impose profound and cascading impacts on mountain socio-ecological systems, from seasonal to multi-annual scales, with downstream repercussions. Early findings reveal that snow droughts can increase melt-out events and reduce snow season duration compared to non-snow-drought years. These changes result in significant hydrological consequences, even in the absence of differences in summer precipitation or air temperature between snow-drought and non-snow-drought years. Beyond hydrology, snow droughts impact vegetation productivity and lead to emergency measures in water-resource management for end users, with effects shaped by the spatial and temporal characteristics of water-supply infrastructure. This study highlights the need to frame snow droughts as a socio-ecohydrological risk, with broad implications for water security in mountain regions and downstream areas.