Changes in the frequency and extremity of rain-on-snow events in the warming climate

Seasonal snowpack significantly influences the catchment runoff and thus represents an important input for the hydrological cycle. A shift from snowfall to rain is expected in the future due to climate changes, as well as changes in the precipitation distribution and intensity. As a result, changes in the frequency and extremity of rain-on-snow events, which are considered to be one of the main causes of floods in winter and spring, may occur.

The objective of this study is 1) to evaluate the frequency, extremity, and trends in occurrence of rain-on-snow events in the past based on existing measurements, and 2) to simulate and evaluate the effect of predicted increase in air temperature on the occurrence of rain-on-snow events in the future. We selected several near-natural mountain catchments in Czechia and Switzerland with significant snow influence on runoff and with available long-time series of daily hydrological and meteorological variables. A semi-distributed conceptual model, HBV-light, was used to simulate the individual components of the water cycle at a catchment scale. The model was calibrated for each of study catchments by using 100 calibration trials which resulted in respective number of optimized parameter sets. The model performance was evaluated against observed runoff and snow water equivalent. Each study catchment was divided into several elevation zones by 100 m, for which all data at a daily resolution were distributed by the model. Rain-on-snow events definition by threshold values for air temperature, rain intensity and snow depth allowed us to analyze inter-annual variations and trends in rain-on-snow events during the study period 1965-2019 and in the future.

The results show that a change of rain-on-snow events related to increasing air temperature differs among individual study catchments and individual elevation zones during winter season. Since both air temperature and elevation seem to be an important rain-on-snow drivers, there is an increasing rain-on-snow events occurrence due to a decrease in snowfall fraction. In contrast, a decrease in total number of events was observed due to the shortening of the period with existing snow cover on the ground. Modelling approach also opened further questions related to model structure and parameterization, specifically how individual model procedures and parameters represent the real natural processes. To understand potential model artefacts might be important when using HBV or similar bucket-type models for impact studies, such as modelling the impact of climate change on catchment runoff.