Making a short story long – the impact of long-term runoff event characteristics on high frequency nutrient transport patterns

Abstract:

Large nutrient loads in rivers threaten aquatic ecosystems and may lead to dead zones in the downstream receiving seas and oceans. A disproportional amount of these nutrients is mobilized and transported during relatively short runoff events. With more frequent extreme weather conditions, due to climate change, the disproportional role of events for nutrient transport might even increase. The advent of high frequency sensors for nutrient concentrations has allowed for detailed analyses of nutrient mobilization and transport mechanisms during runoff events and has highlighted not only the important role, but also revealed large intra- and inter-annual variability of event characteristics. However, the number of events analyzed is often limited and thus little is known about the long-term stability or trends in nutrient transport characteristics. On the contrary, discharge and precipitation measurements have a far longer history of high frequency measurements and allow to trace back runoff event characteristics over decades. Here, we combine these long-term runoff event characteristics with nutrient transport patterns identified in recent high frequency data to analyze the impact of event characteristics on nutrient export and draw conclusions on their development over long time scales and their possible development in the future. To this end, we used a recently developed approach (Tarasova et al., 2020) to characterize and classify runoff events based on daily discharge and precipitation measurements from 1955 – 2017 in 5 contrasting mesoscale catchments in central Germany. In combination with that, we used hourly data on discharge and nitrate concentrations from 2010 – 2017 to relate these long-term event characteristics to nitrate transport patterns, such as the concentration-discharge (CQ) relationship and hysteresis in the event scale CQ behavior. With this time-scale crossing approach, we hope to provide insights not only into the impact of event characteristics on nutrient transport, but also into their long-term stability or possible trends and changes over time.

Tarasova, L., Basso, S., Wendi, D., Viglione, A., Kumar, R., & Merz, R. (2020). A process‐based framework to characterize and classify runoff events: The event typology of Germany. Water Resources Research, 56(5), e2019WR026951.