What affects the hydrological response of rain-on-snow events in low-altitude mountain ranges in Central Europe?

Rain-on-snow (ROS) events influence the hydrological regime of rivers in regions with seasonal snow cover. Although ROS events are often related to floods, they do not always cause severe runoff. During ROS, the snowpack has an ambiguous effect on runoff generation; it can either store a significant portion of rain or amplify runoff by additional snowmelt. There is a need to understand under what circumstances ROS events produce runoff. We analysed eleven years of hourly meteorological, snow water equivalent and streamflow data from 15 catchments located in two mountain ranges in Czechia. We identified 611 ROS events which were further analysed and classified using selected meteorological, snow and runoff indices. The analysis of the runoff response of all ROS events revealed that only 5% of them resulted in high runoff exceeding the 1-year return period, but most of the events (82%) did not cause any significant runoff increase. Employing self-organising maps enabled us to categorise the events and better understand what combination of hydrometeorological characteristics leads to various runoff responses. High volumes of rain together with low snow cover were identified as important factors in the generation of high runoffs. In contrast, a deep and extended snowpack affected by rain under low air temperatures usually caused lower runoffs. The results of this study showed the importance of the snowpack, which can often prevent extreme runoff even when a large amount of rainfall occurs. Understanding the hydrological regime of ROS is becoming even more important with the ongoing decline of the snowfall fraction and subsequent changes in snow storage.

Automated summary

Rain-on-snow events have a significant impact on the hydrological regime of rivers in regions with seasonal snow cover. Analysis shows that only a small percentage of ROS events result in high runoff, while most events do not cause a significant increase in runoff. High volumes of rain and low snow cover are identified as important factors in generating high runoffs.