RESUMO
Pesticides are a major source of pollution for ecosystems. In agricultural catchments, ponds serve as buffer areas for pesticide transfers and biogeochemical hotspots for pesticide dissipation. Some studies have highlighted the specific impact of ponds on the dynamics of pesticides, but knowledge of their cumulative effect at the watershed scale is scarce. Hence, using a modelling approach, we assessed the cumulative role of ponds in pesticide transfer in an agricultural basin (Southwest of France, 1110 km2). The Soil and Water Assessment Tool (SWAT) model was used to model the Save basin, including 197 ponds selected with a Multi-Criteria Decision Aiding Model based on their pesticide interception capacities. The daily discharge, the suspended sediment loads and two herbicide loads (i.e. S-metolachlor and aclonifen) in dissolved and particulate phases were accurately simulated from January 2002 to July 2014 at a daily time step. The presence of ponds resulted in a yearly mean reduction at the watershed outlet of respectively 61 % and 42 % of aclonifen and S-metolachlor fluxes compared to the simulations in the absence of ponds. Sediment-related processes were the most efficient for pesticide dissipation, leading to a mean dissipation efficiency by ponds of 51.0 % for aclonifen and 34.4 % for S-metolachlor. This study provides a first quantification of the cumulative role of ponds in pesticide transfer at the catchment scale in an intensive agricultural catchment.
RESUMO
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 for European rivers. The performance of the Soil and Water Assessment Tool model was assessed in the 1110-km 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, = 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 ( = 0.63). Simulated annual catchment nitrate exportation ranged from 21 to 49 kg ha depending on annual hydrological conditions (average, 36 kg ha). 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 (1998-2010), ranging from 0 to 270 mg L. Nitrate concentration exceeded the European 50 mg L 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 threshold was exceeded.
