Spatial patterns and characteristics of flood seasonality in Europe

In Europe, floods are typically analysed within national boundaries and it is therefore not well understood how the characteristics of local floods fit into a continental perspective. To gain a better understanding at continental scale, this study analyses seasonal flood characteristics across Europe for the period 1960-2010. From a European flood database, the timing within the year of annual maximum discharges or water levels of 4105 stations is analysed. A cluster analysis is performed to identify large-scale regions with distinct flood seasons based on the monthly relative frequencies of the annual maxima. The clusters are further analysed to determine the temporal flood characteristics within each region and the Europe-wide patterns of bimodal and unimodal flood seasonality distributions. The mean annual timing of floods observed at individual stations across Europe is spatially well defined. Below 60 degrees latitude, the mean timing transitions from winter floods in the west to spring floods in the east. Summer floods occurring in mountainous areas interrupt this west-to-east transition. Above 60 degrees latitude, spring floods are dominant, except for coastal areas in which autumn and winter floods tend to occur. The temporal concentration of flood occurrences around the annual mean timing is highest in north-eastern Europe, with most of the floods being concentrated within 1-2 months. The cluster analysis results in six spatially consistent regions with distinct flood seasonality characteristics. The regions with winter floods in western, central, and southern Europe are assigned to Cluster 1 (similar to 36% of the stations) and Cluster 4 (similar to 10 %) with the mean flood timing within the cluster in late January and early December respectively. In eastern Europe (Cluster 3, similar to 24 %), the cluster average flood occurs around the end of March. The mean flood timing in northern (Cluster 5, similar to 8 %) and north-eastern Europe (Cluster 6, similar to 5 %) is approximately in mid-May and mid-April respectively. About 15% of the stations (Cluster 2) are located in mountainous areas, with a mean flood timing around the end of June. Most of the stations (similar to 73 %) with more than 30 years of data exhibit a unimodal flood seasonality distribution (one or more consecutive months with high flood occurrence). Only a few stations (similar to 3 %), mainly located on the foothills of mountainous areas, have a clear bimodal flood seasonality distribution. This study suggests that, as a result of the consistent Europe-wide pattern of flood timing obtained, the geographical location of a station in Europe can give an indication of its seasonal flood characteristics and that geographical location seems to be more relevant than catchment area or catchment outlet elevation in shaping flood seasonality.