The Relative Importance of Different Flood‐Generating Mechanisms Across Europe

Abstract

Inferring the mechanisms causing river flooding is key to understanding past, present, and future flood risk. However, a quantitative spatially distributed overview of the mechanisms that drive flooding across Europe is currently unavailable. In addition, studies that classify catchments according to their flood-driving mechanisms often identify a single mechanism per location, although multiple mechanisms typically contribute to flood risk. We introduce a new method that uses seasonality statistics to estimate the relative importance of extreme precipitation, soil moisture excess, and snowmelt as flood drivers. Applying this method to a European data set of maximum annual flow dates in several thousand catchments reveals that from 1960 to 2010 relatively few annual floods were caused by annual rainfall peaks. Instead, most European floods were caused by snowmelt and by the concurrence of heavy precipitation with high antecedent soil moisture. For most catchments, the relative importance of these mechanisms has not substantially changed during the past five decades. Exposing the regional mechanisms underlying Europe's most costly natural hazard is a key first step in identifying the processes that require most attention in future flood research.

Event‐based classification for global study of river flood generating processes

“…This demonstrates the benefit of focusing on flood processes instead of storm types (cyclone, monsoon, storm). Outside the tropics many studies describe soil moisture as a relevant factor in flood generation for several areas and river basins as well (Berghuijs et al, ; Institute of Hydrology (IoH), ; Lim & Boochabun, ). Our distribution of excess rainfall as dominant flood generating process matches previous studies in the United States (Berghuijs et al, ) and Europe (Berghuijs et al, ).…”

“…If T ( t ) > T crit , where at time t S snow is snow storage (mm), P precipitation input (mm), P rain is liquid precipitation, T is air temperature (°C), T crit is the temperature threshold where rainfall turns to snow (°C), f dd is a melt factor (mm day –1 K –1 ), and P melt is snowmelt rate (mm). f dd is set to 2 mm day –1 K –1 (Berghuijs et al, , 2016). For critical temperature, the data product by Jennings, Winchell, Livneh, and Molotch () were used, which provide global gridded critical temperature for the Northern hemisphere.…”

“…Testing for heavy rainfall conditions before excess rainfall conditions would therefore miss this important flood generating process. Nevertheless, the tree recognizes that wet conditions still require rainfall for flood generation (Berghuijs et al, ) by using an additional decision node, which tests if an above‐average amount of rainfall fell in addition to wet antecedent conditions.…”

“…However, these trends in flood magnitude are not ubiquitous (Petrow & Merz, ) and cannot simply be connected to changes in precipitation (Sharma, Wasko, & Lettenmaier, ). Soil moisture or catchment wetness state often play an important role in flood generation (Berghuijs, Harrigan, Molnar, Slater, & Kirchner, ; Ivancic & Shaw, ; Sharma et al, ; Slater & Villarini, ). For example, Blöschl et al () showed that changes in the seasonal timing of extreme precipitation are not always the clearest explanatory factor for changes in the timing of floods, as in large areas of Europe flood occurrences are more influenced by timing of snowmelt or soil moisture maxima.…”

“…Other input data were kept to a minimum as well, so that a large number of stations could be included in the analysis, and the method is potentially transferable to data‐scarce regions. This is especially valuable, as any prior studies of flood generating processes have been focused on Europe or North America (Berghuijs et al, ; Berghuijs et al, ; Blöschl et al, ). Our study extends the knowledge of flood generating processes to other continents and climates as well.…”