Daily Nitrate Losses: Implication on Long‐Term River Quality in an Intensive Agricultural Catchment of Southwestern France

High nitrate concentrations in streams have become a widespread problem throughout Europe in recent decades, damaging surface water and groundwater quality. The European Nitrate Directive fixed a potability threshold of 50 mg L−1 for European rivers. The performance of the Soil and Water Assessment Tool model was assessed in the 1110‐km2 Save catchment in southwestern France for predicting water discharge and nitrate loads and concentrations at the catchment outlet, considering observed data set uncertainty. Simulated values were compared with intensive and extensive measurement data sets. Daily discharge fitted observations (Nash‐Sutcliffe efficiency coefficient = 0.61, R2 = 0.7, and PBIAS = −22%). Nitrate simulation (1998–2010) was within the observed range (PBIAS = 10–21%, considering observed data set uncertainty). Annual nitrate load at the catchment outlet was correlated to the annual water yield at the outlet (R2 = 0.63). Simulated annual catchment nitrate exportation ranged from 21 to 49 kg ha−1 depending on annual hydrological conditions (average, 36 kg ha−1). Exportation rates ranged from 3 to 8% of nitrogen inputs. During floods, 34% of the nitrate load was exported, which represented 18% of the 1998–2010 period. Average daily nitrate concentration at the outlet was 29 mg L−1 (1998–2010), ranging from 0 to 270 mg L−1. Nitrate concentration exceeded the European 50 mg L−1 potability threshold during 244 d between 1998 and 2010. A 20% reduction of nitrogen input reduced crop yield by between 5 and 9% and reduced by 62% the days when the 50 mg L−1 threshold was exceeded.