Abstract
In 2018–2019, large parts of Europe experienced an unprecedented
multi-year drought with severe impacts on society and ecosystems. This
study is among the first to analyze its impact on water quality by
comparing long-term (1997–2017) nitrate export with 2018–2019 export
in an intensively-monitored mesoscale catchment in Germany. We combined
data-driven analysis of concentration-discharge and load-discharge
relationships with process-based modelling to analyze the catchment
nitrogen retention capacity and the underlying mechanisms of retention
in the soils and during subsurface transport. Within the multi-year
drought, we found a shift in the concentration-discharge relationship at
the catchment outlet, reflecting exceptionally low riverine nitrate
concentrations during dry periods and exceptionally high concentrations
during subsequent wet periods. Nitrate loads during the multi-year
drought were up to 70% higher than expected from the long-term
relationship between discharge and loads. Model simulations suggested
that this increase was driven by a decrease in denitrification and plant
uptake in exceptionally dry soils and subsequent flushing of accumulated
nitrogen during rewetting via fast, shallow flow paths. As a
consequence, the overall capacity of the catchment to retain nitrogen
was reduced, which was confirmed by model results for nitrate in the
soil leachates. This observation was most evident in the upstream
sub-catchments, which have relatively short transit times during wet
periods (<2 months). Downstream, longer transit times
(>20 years) inhibit a fast response of riverine water
quality to drought conditions, which might result in a long-term drought
legacy becoming visible in the future. Overall, our study reveals that a
severe multi-year drought threatens water quality by intensifying
nitrate pollution. This is crucial knowledge for water quality
management in the face of climate change, as such droughts are predicted
to become more frequent and prolonged across Europe.