Assessing the Effects of Climate Change Projections on Water Fluxes and Streamwater NO ₃ –N in Lowland Catchments Using Isotope‐Aided Water Quality Modeling

Abstract

Understanding how climate change alters hydrological connectivity and associated nitrate dynamics remains a key challenge in lowland, mixed land‐use catchments. Here, we use a tracer‐aided, fully distributed ecohydrological modeling framework to investigate how future climate conditions may influence water fluxes and nitrate dynamics in such systems, using the Demnitzer Mill Creek catchment in Germany as an exemplar. To provide mechanistic insight into hydrological transport and biogeochemical processing, we applied diagnostic metrics based on water age and the Damköhler (Da) number, which links water residence times with reaction timescales. Results indicate that projected warming and a shift of precipitation toward winter progressively strengthen hydrological connectivity, leading to increased infiltration, evapotranspiration, and subsurface flow contributions. This stronger connectivity reorganizes nitrate transport across the catchment, with enhanced lateral redistribution, declining agricultural soil nitrate stocks, and lower stream nitrate concentrations. At the same time, processing times decrease more strongly than hydrological exposure times, increasing Da number and indicating a shift toward stronger reaction dominance. Uncertainty arises from climate forcing projections, simplified representations of subsurface processes, and assumptions regarding future agricultural practices. Overall, the results highlight the coupled role of transport and processing of nutrients and the value of coupled tracer‐aided water quality models. The findings also emphasize hydrological connectivity as a key control on future water quality.