Evaluation of concentration-discharge dynamics and nitrogen export on anthropogenic inputs and stormflow across alternative time-scales

Impacts of agricultural activities and hydrologic variables on nitrogen (N) loads have been investigated previously, but interactions among these factors are still poorly addressed across alternative time-scales. The concentration-discharge (C-Q) dynamics at short time-scales could be useful to disentangle the impacts of agricultural activities because it can provide more information about the interaction between N export and input. In this study, total dissolved N (TDN) concentrations/fluxes-discharge dynamics across different time scales (annual, daily and hourly) were used to investigate the potential influences of agricultural activities and the interaction with hydrologic variables. At the long-term scale, streamflow and instream TDN concentrations exhibited the same inter-annual variations. At daily scale, higher temporal variations of streamflow (compared to instream TDN concentrations) occurred during stormflow-dominated periods. It highlighted the importance that stormflow may have substantial impact over N concentration changes over time. At the smaller time scale, direct influence of fertilizer application on watershed N export was unraveled by the difference in stream TDN concentrations and export fluxes between the early growing period (with more fertilizer application) and the later period (with less). In addition, the differences may increase with increasing flow conditions, which indicated that the impact of agricultural activities could be affected by hydrologic variables. These findings provide important information for improved best management practices (BMPs) to reduce N pollution from nonpoint sources (NPS).