Flood drivers and trends: a case study of the Geul River Catchment (the Netherlands) over the past half century

Abstract. Extreme precipitation in July 2021 caused devastating flooding in Germany, Belgium and in the Netherlands, particularly in the Geul river catchment. Such precipitation extremes were not recorded previously and were not expected to occur in summer. This contributed to poor flood forecast and hence to large damage. Climate change was mentioned as a potential explanation for these unprecedented events. Yet, before such a statement can be made, we need a better understanding of the drivers of floods in the Geul and their long-term variability, which are poorly understood and have not been examined recently. In this paper, we use an event-based approach to identify the dominant flood drivers in the Geul and employ a multi-temporal trend analysis to investigate their temporal variabilities, as well as, a novel methodology to detect the dominant direction of a trend. Results suggest that extreme 24-hour precipitation cannot solely lead to floods. Heavy multi-day precipitation is the primary high-flow driver in the catchment and the joint probability of heavy and prolonged rainfall with wet initial conditions (compound event) determines the chances of flooding. Critical precipitation (precipitation that leads to floods) shows a consistent increase in the winter half-year, a period in which more than 70 % of extremely high flows have occurred historically. While no consistent trend patterns are evident in the majority of precipitation and extreme flow trends in the summer half-year, an increasing direction in the recent past is visible.