A Damköhler-based catchment nitrate transport-processing integration and its responses to droughts

Catchment-scale nitrate dynamics involve complex interactions coupling hydrological transport and biogeochemical transformations, imposing challenges for source control of diffuse pollution. Here, we propose a novel spatio-temporal framework for catchment-scale quantification of Damköhler number (Da) based on the ecohydrological modelling platform EcH₂O-iso and a catchment nitrate module. We examined Da variability of the dominant process of denitrification and N removal in the intensively instrumented, heterogeneous Selke catchment (456 km², central Germany). Results showed that warm-season N losses from denitrification was of catchment-wide significance (Da >1), while its high spatial variations were co-determined by varying exposure times (e.g., hydrologically isolated areas with long residence times and old water) and removal efficiencies (e.g., hotspots of channel-connected lowland areas). Moreover, Da demonstrated a systematic shift to transport-dominance during the wet-spring season (from >1 to <1). Under the prolonged 2018-2019 droughts, denitrification removal generally reduced, resulting in further N accumulation in agricultural soils. Besides, the hydrologically disconnected lowland areas (with high water ages) exhibited extra risks of groundwater contamination. Importantly, the channel-connected lowlands exhibited high removal efficiencies, as well as high resilience to the disturbances like the droughts. These insights into integrated catchment functioning highlighted important management implications for nature-based, spatially targeted mitigation measures.