Assessment of Spatial and Temporal Nitrate Dynamics in a Medium‐Sized Watershed Using <scp>SWAT</scp> ‐ <scp>RIVE</scp>

ABSTRACT Quantifying nitrate fluxes at the watershed scale is essential in human‐impacted areas to understand ecosystems' adaptation to pollution. Indeed, it is important to understand the quantity that can be naturally eliminated by continental waters and wetlands to anticipate pollution risks or to adapt land management policies, such as through the rehabilitation of watercourses, wetlands, or by limiting nitrogen (N) inputs. This study aims to characterize and quantify nitrate fluxes and their regulation in the Vienne watershed in both terrestrial and aquatic ecosystems. The catchment‐scaled SWAT‐RIVE model was applied at a daily time step for the period 1993–2017 to assess spatiotemporal dynamics of nitrate in all the hydrological compartments. Of the N fluxes reaching the soil nitrate pool, SWAT‐RIVE simulates average soil denitrification of 20 kgN ha −1 y −1 for the entire study period, removing 14.6% of the incoming flux to soils. The highest denitrification rates are in agricultural soils due to high nitrate inputs and wetlands due to higher water content and organic carbon concentration. A substantial part of the inputs to soils is transferred to the river system (8.5%). In‐stream retention is 0.20 kgN ha −1 y −1 (1.7% of total inputs to streams) with benthic denitrification being the dominant process (54%), followed by biofilm consumption (38%). Regarding temporal trends, nitrate diffuse inputs do not show a significant trend, except for the deep aquifer groundwater flow, which decreases slightly. Thus, the river plays a limited but essential role in regulating water quality. Certain factors of advanced anthropogenic impacts, such as excessive nitrate inputs and low organic carbon levels, could explain the low functioning of riverine denitrification. These results support knowledge of the significant imbalance between the N supplied by continental surfaces to continental waters compared to what the latter can manage.