Rain-on-snow runoff events in mountainous catchments under climate variability and change

Mountains are sensitive to the increase in air temperature because it causes a shift from snowfall to rainfall, resulting in a decrease in snow storage. This further affects the runoff regime, including runoff extremes such as rain-on-snow (RoS) events and floods. In this study, we attributed these climate changes to simulated variations in RoS events using a sensitivity analysis of precipitation and air temperature and evaluated the subsequent effects on ROS-related runoff responses. We selected 93 catchments in Czechia and Switzerland, representing both high alpine and rain-snow transition areas. We used a conceptual catchment model to simulate snow storage and runoff for the reference historical period and for the ensemble of 24 climate perturbations reflecting the expected increase in air temperature and changes in precipitation.

Changes in RoS due to climate change were highly variable between regions, between elevations and within the cold season, with RoS occurring most frequently at elevations between 1000 and 2000 m a.s.l. RoS days are expected to become less frequent with future increases in air temperature, especially at lower elevations. The +4°C perturbation suggested a decrease in RoS days by about 75 % for the Czech catchments. In contrast, the Swiss catchments may respond less sensitively, with the number of RoS days even increasing, especially during the winter months and at higher elevations, which may be further enhanced by increased precipitation. The contribution of RoS events to total annual runoff is expected to decrease from 10% to 2-4% for the +4°C perturbation in the Czech catchments and from 18% to 5-9% in Switzerland. However, the contribution of RoS to runoff may increase in winter months, especially for climate perturbations leading to an increase in precipitation, demonstrating the joint importance of air temperature and precipitation for future hydrological behaviour in snow-dominated catchments. The results have important implications for climate change adaptation strategies, such as water management, flood and drought protection, or hydropower.