When does nitrate peak in rivers and why? Catchment traits and climate relate to synchrony with discharge

Abstract. Anthropogenic nitrogen loading has disrupted riverine biogeochemical cycles, degrading water quality and altering ecosystem functions. Rivers mediate nitrogen transport and reactivity, yet at the seasonal scale, the temporal links between peak river nitrate concentrations (N) and water flow (Q) are poorly understood. Here, we reconstructed daily nitrate concentrations from routine monitoring data using Weighted Regressions on Time, Discharge, and Season (WRTDS). We assessed long-term N–Q synchrony and its variability across 66 English catchments (2000–2019) and used a Random Forest model to help identify climatic, hydrological, and anthropogenic controls. This revealed three general behaviours: (1) smaller catchments dominated by agriculture displayed peak N during high flow (QMax-Synced, 28.8 % of catchments), (2) larger and/or more urbanised catchments had the highest N concentrations during low flow periods consistent with point-source dominance (QMin-Synced, 25.8 % of catchments), and, (3) larger highly mixed land use catchments displayed a decoupling of N and flow conditions, i.e. were asynchronous (Asynced, 45.5 % of catchments). The temporal consistency of peak N–Q synchrony was determined by the dominant hydrological processes and their interaction with anthropogenic pressures. In QMax-Synced catchments, wetter winters enhanced hydrological connectivity, mobilising diffuse nitrate and reinforcing high-flow synchrony. In QMin-Synced catchments, synchrony reflected the dominance of urban point-source inputs (represented as urban area and population density) but was sustained only under sufficiently extreme low flows. Asynced catchments showed the greatest year-to-year switching, reflecting sensitivity to hydroclimatic variability that intermittently favoured QMin- or QMax-like behaviour. Our findings reveal that nitrate–discharge synchrony is not fixed but dynamically regulated by hydroclimatic variability, catchment connectivity, and human infrastructure. Framing nitrate export through synchrony exposes a critical temporal dimension of nutrient cycling that purely spatial analyses of loads or concentrations would overlook, providing new insight into how climatic and anthropogenic forcing interact to shape water-quality responses in human-modified landscapes.