A multi-scale analysis of nitrate concentration-discharge relationships in a small agricultural watershed

Authors: MARSH, MELINDA - Pennsylvania State University; DUNCAN, JON - Pennsylvania State University; Buda, Anthony; Kennedy, Casey; Millar, David; HORNER, KEVIN - Pennsylvania State University; Reiner, Michael

Submitted to: Water Resources Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/9/2026
Publication Date: 7/11/2026
Citation: Marsh, M., Duncan, J., Buda, A.R., Kennedy, C.D., Millar, D.J., Horner, K., Reiner, M.R. 2026. A multi-scale analysis of nitrate concentration-discharge relationships in a small agricultural watershed. Water Resources Research. 62(7).Article e2025WR043170. https://doi.org/10.1029/2025WR043170.
DOI: https://doi.org/10.1029/2025WR043170

Interpretive Summary: Identifying the processes that control nitrate transport is central to managing nitrogen losses from agriculture. In this study, we installed state-of-the-art nitrate sensors at two streamflow monitoring sites in a small agricultural watershed. One site was located on a headwater stream, while the second site was located downstream near the watershed outlet. At each site, we monitored streamflow and nitrate every 30 minutes for three years. Using simple metrics, we summarized relationships between nitrate and flow over the entire record, as well as for 97 storms that were monitored simultaneously at both sites. While nitrate increased with flow in both streams over the long term, metrics for individual storm events suggested more complex behavior that was affected by season, wetness levels, and rainfall amount. Generally, we found that as flows increased with rainfall, nitrate declined. After rainfall ended, nitrate increased again as flows receded, often rising to levels that were higher than what was measured before rainfall began. This pattern indicated a delayed groundwater nitrate source. Findings suggest that a more complete picture of nitrate fate and transport can be gained when metrics summarizing associations between nitrate and flow are assessed at multiple sites during storms and over longer periods.

Technical Abstract: Applying a nested watershed framework to concentration-discharge (cQ) relationships offers valuable insights into the spatial and temporal variability of solute transport. We examined nitrate-N cQ relationships at two nested spatial scales in a small agricultural watershed using three years of 30-minute data, analyzing both composite records and individual storm events. Metrics including the flushing index (FI), hysteresis index (HI), cQ slope (b) and the ratio of coefficients of variation for concentration and discharge (CVC/CVQ) were used to compare event-scale and composite cQ patterns, evaluate the influence of metric choice, assess spatial consistency during 97 concurrent storm events, and identify controls on inter-event variability. Composite-scale analyses showed nitrate enrichment, while event-scale patterns were dominated by dilution or chemostasis, depending on the metric used. Across scales, nitrate-N enrichment on the falling limb, or post-storm enrichment, emphasized the need for an additional metric to characterize falling limb dynamics. Spatial consistency in cQ behavior varied by metric: CVC/CVQ and FI showed higher consistency during concurrent storms, while cQ slope and HI often revealed contrasting patterns. Controls on event-scale cQ patterns varied across spatial scales. Indicators of seasonality, antecedent moisture, and storm event magnitude influenced the FI and cQ slope only at the subcatchment, while the HI was primarily driven by storm magnitude across scales. These findings highlight the importance of temporal scale, spatial scale, and metric choice in interpreting nitrate-N export dynamics.