Trend analysis of the runoff patterns in Central and Northern Europe: Identification of changes in magnitude and timing

Abstract

In today's world, extreme flooding and drought events are becoming increasingly common globally. These challenges arise from various factors, with climate change and changes in land use being among the most significant contributors.

While numerous studies have explored the combined impact of climate change and land use on streamflow, there is a research gap when it comes to analyzing historical data for changes in the magnitude and timing of discharge peaks and low-flow periods. To address this gap, this MSc Thesis investigates river discharge in five European countries: Belgium, Germany, France, Luxembourg, and the Netherlands.

To analyze potential patterns and variations in magnitude changes, trend analyses were conducted for annual and monthly mean daily flows. Non-parametric methods such as Sen's slope and the Mann-Kendall test were employed to calculate trends in average, maximum, and minimum daily flows at both yearly and monthly levels. The issue of autocorrelation in discharge flows was also addressed by using a modified version of the Mann-Kendall test for stations with autocorrelated data. Furthermore, the possible shifts in the timing of discharge peaks and low-flow periods were examined. We employed statistical tools such as statistical entropy, Kullback-Leibler divergence, and various descriptive statistics to determine if there have been changes in the month with the highest flow over the years.

The study's results generally align with existing research. Regarding annual discharges, for average and maximum analyses, stations with decreasing trends were predominantly found in the North, East, and central parts of the study area (Germany), while the North-West exhibited stations with significant increasing trends in most cases (North France).

In yearly minima discharge flows, the patterns were aligned with average and maximum analyses; however, additional stations showed decreasing trends, which were located in Belgium. In the monthly analysis, positive trends were primarily observed during winter months (February, December, and January), while April and March showed decreasing trends in most cases (monthly average and maxima analyses), with a few exceptions in minimum daily flows.

Notably, more than 50% of the stations exhibited shifts in the month when they experienced maximum and minimum discharge, particularly between 1980-2000 and 2000-2021. This finding suggests potential avenues for future research.