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

Abstract. Anthropogenic nitrogen loading has disrupted global 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 used the approach of Weighted Regressions on Time, Discharge, and Season (WRTDS) to reconstruct daily timeseries of N concentrations from routine monitoring data. These were used to assess the long term N-Q synchrony and its variability across 66 river catchments in England (2000–2019), and a Random Forest Model was used to identify the key controls on each synchrony type. 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 likely due to point-source inputs (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, 46.8 % 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, and steeper slopes promoted more rapid runoff, reinforcing 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 in the dominant synchrony year type, with wetter years likely enhanced groundwater recharge and legacy-N delivery, favouring QMin-like behaviour, whereas years dominated by rapid runoff and shallow flow paths promoted QMax-like winter flushing. 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 a 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.