Hydrological controls on nutrient dynamics and load estimation in an agricultural watershed: a case study in Rock Creek, Ohio

Abstract

This study investigates the spatiotemporal variability of nutrient concentrations (soluble reactive phosphorus, SRP; total phosphorus, TP; and nitrate, NO₃⁻) and their relationships with hydrological conditions in the Rock Creek watershed, a representative agricultural basin in northwestern Ohio. High-temporal-resolution water quality and discharge data were analyzed over a two-year period (2010–2011) to elucidate seasonal trends, flow-nutrient correlations, and to compare nutrient load estimation methods. The results demonstrate that nutrient transport in the watershed is strongly event-driven, with SRP and TP concentrations peaking during high-flow periods (spring snowmelt and summer/fall storms), exhibiting significant positive correlations with discharge (log-transformed R² = 0.27–0.47). In contrast, nitrate concentrations displayed a distinct seasonal pattern, with a spring flush followed by declining concentrations, suggesting depletion of legacy storage. Notably, flow-weighted mean concentrations (FWMC) exceeded arithmetic means by 2–3 times, revealing that conventional averaging methods underestimate nutrient loads during high-flow events. These findings emphasize the dominance of non-point source pollution during episodic runoff events and underscore the need for targeted management strategies to mitigate nutrient pollution in similar agricultural watersheds.