Abstract
In 2018–2019, Central Europe experienced an unprecedented multi-year
drought with severe impacts on society and ecosystems. In this study, we
analyzed the impact of this drought on water quality by comparing
long-term (1997-2017) nitrate export with 2018–2019 export in a
heterogeneous mesoscale catchment. We combined data-driven analysis with
process-based modelling to analyze nitrogen retention and the underlying
mechanisms in the soils and during subsurface transport. We found a
drought-induced shift in concentration-discharge relationships,
reflecting exceptionally low riverine nitrate concentrations during dry
periods and exceptionally high concentrations during subsequent wet
periods. Nitrate loads were up to 70% higher compared to the long-term
load-discharge relationship. Model simulations confirmed that this
increase was driven by decreased denitrification and plant uptake and
subsequent flushing of accumulated nitrogen during rewetting. Fast
transit times (<2 months) during wet periods in the upstream
sub-catchments enabled a fast water quality response to drought. In
contrast, longer transit times downstream (>20 years)
inhibited a fast response but potentially contribute to a long-term
drought legacy. Overall, our study reveals that severe multi-year
droughts, which are predicted to become more frequent across Europe, can
reduce the nitrogen retention capacity of catchments, thereby
intensifying nitrate pollution and threatening water quality.