Observed Impact of Spatial and Temporal Variability of Snowmelt Energy Balance Factors and Runoff Sources During Rain on Snow Events

Abstract:

Rain on snow (ROS) floods are occurring in many montane catchments in temperate climates in many parts of the world. These flood events have been proven to be notoriously hard to predict due to their complex nature. Furthermore due to predicted higher future winter temperatures and precipitation amounts, the potential for ROS floods will likely increase even more in the coming years. Two ROS events in a 40 km² basin in the southwestern part of Germany (one resulting in a flood with a 20 year return period) were intensively observed and analyzed using data from 30 combined climate and snow observation stations (SnoMoS) within the basin. The data was used to calculate a detailed energy balance and showed the large spatial variability present especially in the turbulent fluxes which dominated the snowmelt energy balance during both events. The observed energy balance components also showed that absolute melt energy amounts were always positive and very similar for forested and open areas indicating that water from snowmelt was available to runoff continuously and from the entire basin throughout the ROS events contributing to the extensive flooding. A spatial analysis of how much water was generated from separate parts of the basin throughout the entire ROS events showed the crucial importance the pre-event snow cover conditions have on the flood formation with the snow cover either acting as an initial buffer or as an accelerant to flood runoff depending on snow temperature and initial snow retention storage capacity. An analysis of 14 additional ROS events observed over a three year period showed that snowmelt contributed on average 48% of total water available for runoff available with a maximum of 68%. Overall the study proved the absolutely crucial importance that the snow cover has on the timing and extent of ROS floods.