The effect of hydrological events on stream nitrate dynamics at the catchment scale

Abstract

Nitrate contamination of freshwater systems is a major global concern for ecosystems and drinking water supply. This thesis investigates how hydrological events control nitrate mobilization across catchments. First, drought and post-drought effects were analyzed in 182 German catchments using low-frequency nitrate data. Droughts generally reduced nitrate concentrations, while winter droughts and post-drought periods showed contrasting responses linked to climate and catchment retention capacity. Second, runoff events in 184 catchments were examined using deviations from concentration-discharge relationships. Snow-influenced and highly connected events enhanced nitrate mobilization, whereas rainfall events following dry conditions reduced it. Third, a deep learning model applied to nine U.S. catchments showed that spatial soil moisture patterns improve nitrate prediction. Overall, hydrological connectivity and antecedent conditions are key controls on nitrate dynamics.