Improving the understanding of N transport in rural catchments under Atlantic climate conditions from analysis of the concentration-discharge relationship derived from a high frequency data set

Abstract

Abstract. Understanding processes controlling stream nutrient dynamics over time is crucial for implementing effective management strategies to prevent water quality degradation. In this respect, the study of the nutrient concentration-discharge (C-Q) relationship during individual runoff events can be a valuable tool for extrapolating the hydrochemical processes controlling nutrient fluxes from streams. This study investigated nitrogen concentration dynamics during events by analyzing and interpreting the nitrogen C-Q relationship in a small Atlantic (NW Iberian Peninsula) rural catchment. To this end, nitrate (NO3) and total Kjeldahl nitrogen (TKN) concentrations were monitored at high temporal resolution during 102 runoff events over a 6-year period. For each of the selected runoff events, C-Q response was examined visually for the presence and direction of hysteresis loops and classified into three types of responses: clockwise and anticlockwise and no hysteresis. Some metrics, such as the change in concentration (ΔC) and the overall dynamics of hysteresis loops (ΔR), were used to quantify nitrogen behavior during the runoff events. The results showed how transport mechanisms varied between parameters. The most frequent hysteretic response for NO3 was enrichment with anticlockwise rotation, indicating that subsurface flow is the main pathway to the stream. On the contrary, the TKN dynamic was dominated by clockwise hysteresis, suggesting that surface runoff is mainly responsible for the transport of TKN to the river. Hysteresis direction (ΔR) and magnitude (ΔC) were better explained by event characteristics, such as rainfall, runoff, and discharge increase than by antecedent conditions (antecedent precipitation and baseflow).