Identifying nitrate transport mechanisms and export dynamics using water age modeling and concentration-discharge relationships
Tile-drained agricultural catchments in the Midwestern U.S. are a significant source of nitrate (NO3-) loss to surface waters. Water flow paths and residence times determine the length of contact between NO3- and sub-surface material, controlling the rate of biogeochemical reactions and nitrogen removal via denitrification. In this project, we assessed the coupled transport of water and NO3- during storm events. Using a StorAge Selection-based water age model, we partitioned river discharge into three flow components to investigate catchment-scale water mixing and preferential removal. Additionally, we used paired high-frequency NO3- concentration-discharge data to identify NO3- export and hysteresis patterns. Predominantly, NO3- showed mobilization export and anti-clockwise hysteresis during events, indicating that it is transport limited. Flow from riparian areas significantly contributed to low-flow summer/fall events, while overland and sub-surface fast-flow pathways were activated only during extreme-flow winter/spring events. Overall, initial storm NO3- concentrations significantly correlate with the predominant event flow components.
