Increasing occurrence of high-intensity rainstorm events relevant for the generation of soil erosion in a temperate lowland region in Central Europe

Rainfall times series with a high temporal resolution of 1 min for the Emscher–Lippe catchment area were analysed for trends in regard to erosion-relevant storm events with intensities larger than 0.3 mm/min (>20 mm/h) for the last 70 years using the Mann–Kendall test. The analysis data showed for all investigated stations an increase of rainstorm events with erosion-relevant rainfall intensities; the trend was more pronounced over the last 35 years and was shown to be statistical significant for a critical zone with rainfall characteristics of: intensity iRain = 0.3–0.7 mm/min, duration tRain = 2–10 min and depth hRain = 1–3 mm. The increase of rainstorm events over the last 35 years was more pronounced in the summer season (July–September), but increases were detected also in the other seasons. The high-intensity storm events occured only ca. 4–15 times/year; the estimated trend increases of up to 0.5 events/year could therefore result in a multiplication of the occurrence of these storm events within only a few decades. This study has shown for the first time that the rainfall regime for temperate lowland regions in Germany has changed within its erosion-relevant spectrum over the last couple of decades. Parallel to the changes in the rainfall regime, increases in the annual and seasonal average temperature and changes in the occurrence of circulation patterns responsible for the generation of high-intensity storms were found, however not in the occurrence of thunderstorms. A climate-change driven increase of rain storm events over the last couple of decades and a potential future continuation of this trend may have several adverse effects for the study area and potentially for a larger region in Central Europe, for example an increase of soil erosion on slopes, the depletion of soil nutrients, an increased transport of nutrients and contaminants attached to sediment particles, their transfer into surface water bodies and consequential negative effects on stream habitats.