Abstract
<strong class="journal-contentHeaderColor">Abstract.</strong> 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 (NO<sub>3</sub>) 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 NO<sub>3</sub> 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).
Highlights
Water is essential to life and, water resources are at the core of the development of societies and ecosystems. 30 anthropogenic activities have altered the natural state of this valuable resource, affecting the quantity and quality of water, as well as the health of the aquatic ecosystem (Meybeck, 2005; Vörösmarty et al, 2010; Abbot et al, 2019)
The total Kjeldahl nitrogen (TKN) dynamic was dominated by clockwise hysteresis, suggesting that surface runoff is mainly responsible for the transport of TKN to the river
4 Discussion 4.1 Most frequent N runoff event response from hysteresis interpretation In general, the NO3 and TKN concentrations increased during runoff events in comparison to baseflow conditions, indicating 295 the predominance of an enrichment response during runoff events and suggesting that the N delivery in the catchments is mainly controlled by diffuse sources
Summary
Water is essential to life and, water resources are at the core of the development of societies and ecosystems. 30 anthropogenic activities have altered the natural state of this valuable resource, affecting the quantity and quality of water, as well as the health of the aquatic ecosystem (Meybeck, 2005; Vörösmarty et al, 2010; Abbot et al, 2019). Water quality assessments historically have relied on routine low-frequency monitoring at main rivers, commonly every two weeks or at monthly resolution This traditional sampling method can provide valuable 40 information to identify sites that are under pressure due to anthropogenic activities, to observe long-term trends in relation to land use, but cannot provide knowledge on nutrient dynamics under contrasting hydrological conditions, which is essential to develop suitable management programs to restore or maintain water quality (Bieroza et al, 2018). Accurate assessments of the complex processes of N dynamics in these systems are hindered by 65 the limited availability of high-frequency data This information is essential to anticipate changes in freshwater resources in compliance with the Water Framework Directive planning and monitoring norms. It is necessary to provide new information on the issue to augment current studies across Europe and the Iberian Peninsula, in particular
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