Transformation of Generation Processes From Small Runoff Events to Large Floods

Abstract

Mixture of runoff generation processes poses a challenge for predicting upper flood quantiles. We examined transformations of generation processes from all identifiable runoff events to frequent and upper tail floods for a large set of mesoscale catchments and observed a substantial change of the dominant processes. Two trajectories of transformation were detected. In regions where floods occur almost exclusively in winter the dominance of processes related to snowmelt consistently increases from small events to frequent and upper tail floods. In catchments characterized by frequent winter‐spring floods and occasional summer‐autumn flood events triggered by rare meteorological phenomena (e.g., Vb cyclones), processes that dominate upper tails are not adequately represented in the sample of frequent floods. Predictions of extremes and projections of flood changes might remain highly uncertain in the latter cases.

Plain Language Summary: Prediction of floods remains a challenging task for the engineering practice. Floods triggered by different physical mechanisms have contrasting statistical attributes. Mixture of these processes hinders reliable prediction of the largest floods. In this study we classified a large number of streamflow events observed in a wide range of German river basins according to their generation processes. We analyzed changes in the frequency of occurrence of different generation processes, from the smallest identifiable runoff events to annual floods to rarer events corresponding to larger river flows. Interestingly, for some river basins certain processes tend to consistently increase their frequency from small streamflow events to common and larger floods. In other cases, we observed an opposite tendency. Certain processes become less important for the generation of annual floods compared to small streamflow events but then dominate generation of the largest floods. This has important implications for our ability to predict extreme floods and their possible changes.

Key Points:

We analyzed transformation of processes from small runoff events to larger floods using a process‐based framework for event characterization. A substantial transformation of the frequency of processes from small runoff events to frequent and upper tail floods is observed. Differences in trajectories of process transformation among catchments suggest regionally variable predictability of extremes.