In July 2021, extreme precipitation caused devastating flooding in Germany, Belgium and the Netherlands, particularly in the Geul River catchment. Such precipitation extremes had not been previously recorded and were not expected to occur in summer. This contributed to poor flood forecasting and, hence, extensive damage. Climate change was mentioned as a potential explanation for these unprecedented events. However, 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 recently examined. In this paper, we use an event-based approach to identify the dominant flood drivers in the Geul. We also employ (1) a multi-Temporal trend analysis to investigate their temporal variability and (2) a novel methodology to detect the dominant direction of any trend. Results suggest that extreme 24 h precipitation alone is typically insufficient to cause floods. The joint probability of extreme and prolonged rainfall combined with wet initial conditions (compound event) determines the chances of flooding. Flood-producing precipitation shows a consistent increase in the winter half-year, a period in which more than 70 % of extremely high flows have historically occurred. While no consistent trend patterns are evident in the majority of precipitation and extreme flow trends in the summer half-year, an increasing direction is visible in the recent past.

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The Geul river, a tributary of the Meuse located in the south Netherlands, Belgium, and part of Germany, is vulnerable to extreme hydrological events, such as floods, droughts, and erosion. There is evidence that these conditions are exacerbated by land use and climate changes. This thesis focuses on investigating the temporal variability and trends of hydrometeorological variables, as well as the main land use changes in the Geul river catchment. Accordingly, this thesis aims to improve understanding of the impacts of climate change and human intervention on the hydrological response of the Geul. In the first part of the thesis, temporal changes in (extreme) precipitation regimes for the period 1951-2021 and potential evaporation for the period 1965-2021, are identified by calculating trends for each 30-year moving period within the available time frames. The main land use changes are evaluated using CORINE annual land cover maps for the period 1990-2018. Changes in discharge regimes are investigated using a multi-temporal approach in which trends are evaluated in every possible combination of start and end years. In the second part of the thesis, the effects of climate variability and land use changes on the variability of runoff patterns are assessed. This is achieved by correlating the rates of change between extreme precipitation and maximum discharges all over the moving periods. The work also includes the use of a water balance separation framework to estimate the attribution of alterations in mean stream flows to land use and climate changes. A statistically significant increase in very wet days is reported in the area that mainly derives from the winter period. The extreme summer precipitation
shows a relatively strong increase since the 1980s. The trend analysis in the potential evaporation time series suggests a very strong and stable increase. The main land use changes dominated before the 1970s, while there do not appear to be significant changes in the catchment from 1990 to 2018. Extreme annual discharges show an increasing insignificant tendency, while mean stream flows are decreasing. Results suggest that the variability of extreme and mean flows is mainly driven by climate variability, while at the same time the effects of land use changes on runoff patterns are not visible for the period 1970-2021. This study emphasizes that climate change should be incorporated into flood designs and climate adaptation strategies should be provided.