Characterizing Catchment-Scale Nitrogen Legacies and Constraining their
Uncertainties
Abstract
Improving nitrogen (N) status in European water bodies is a pressing
issue. N levels depend not only on current but also past N inputs to the
landscape, that have accumulated through time in legacy stores (e.g.
soil, groundwater). Catchment-scale N models, that are commonly used to
investigate in-stream N levels, rarely examine the magnitude and
dynamics of legacy components. This study aims to gain a better
understanding of the long-term fate of the N inputs and its
uncertainties, using a legacy-driven N model (ELEMeNT) in Germany’s
largest national river basin (Weser; 38,450 km2) over the period
1960-2015. We estimate the nine model parameters based on a progressive
constraining strategy, to assess the value of different observational
datasets. We demonstrate that beyond in-stream N loading, soil N content
and in-stream N concentration allow to reduce the equifinality in model
parameterizations. We find that more than 50% of the N surplus
denitrifies (1480-2210 kg ha-1) and the stream export amounts to around
18% (410-640 kg ha-1), leaving behind as much as around 230-780 kg ha-1
of N in the (soil) source zone and 10-105 kg ha-1 in the subsurface. A
sensitivity analysis reveals the importance of different factors
affecting the residual uncertainties in simulated N legacies, namely
hydrologic travel time, denitrification rates, a coefficient
characterising the protection of organic N in source zone and N surplus
input. Our study calls for proper consideration of uncertainties in N
legacy characterization, and discusses possible avenues to further
reduce the equifinality in water quality modelling.