Disentangling the impact of catchment heterogeneity on nitrate export
dynamics from event to long-term time scales
Abstract
Defining effective measures to reduce nitrate pollution in heterogeneous
mesoscale catchments remains challenging if based on concentration
measurements at the outlet only. One reason is our limited understanding
of the sub-catchment contributions to nitrate export and their
importance at different time scales. While upstream sub-catchments often
disproportionally contribute to runoff generation and in turn to
nutrient export, agricultural areas, which are typically found in
downstream lowlands, are known to be a major source for nitrate
pollution. To disentangle the interplay of these contrasting drivers of
nitrate export, we analyzed seasonal long-term trends and event dynamics
of nitrate concentrations, loads and the concentration-discharge
relationship in three nested catchments within the Selke catchment (456
km²), Germany. The upstream sub-catchments (40.4 % of total catchment
area, 34.5 % of N input) had short transit times and dynamic
concentration-discharge relationships with elevated nitrate
concentrations during wet seasons and events. Consequently, the upstream
sub-catchments dominated nitrate export during high flow and
disproportionally contributed to overall annual nitrate loads at the
outlet (64 %). The downstream sub-catchment was characterized by higher
N input, longer transit times and relatively constant nitrate
concentrations between seasons, dominating nitrate export during low
flow periods. Neglecting the disproportional role of upstream
sub-catchments for temporally elevated nitrate concentrations and net
annual loads can lead to an overestimation of the role of agricultural
lowlands. Nonetheless, in agricultural lowlands, constantly high
concentrations from nitrate legacies pose a long-term threat to water
quality. This knowledge is crucial for an effective and site-specific
water quality management.