No abstract

Distributed models and a good knowledge of the catchment studied are required to assess mitigation measures for nitrogen (N) pollution. A set of alternative scenarios (change of crop management practices and different strategies of landscape management, especially different sizes and distribution of set-aside areas) were simulated with a fully distributed model in a small agricultural catchment. The results show that current practices are close to complying with current regulations, which results in a limited effect of the implementation of best crop management practices. The location of set-aside zones is more important than their size in decreasing nitrate fluxes in stream water. The most efficient location is the lower parts of hillslopes, combining the dilution effect due to the decrease of N input per unit of land and the interception of nitrate transferred by sub-surface flows. The main process responsible for the interception effect is probably uptake by grassland and retention in soils since the denitrification load tends to decrease proportionally to N input and, for the scenarios considered, is lower in the interception scenarios than in the corresponding dilution zones.

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Climate change and increased atmospheric CO2 concentration can impact hydrological and nitrogen cycling at the catchment scale. The objective of this study is to assess these impacts in an intensive agricultural headwater catchment in western France. A calibrated and validated agro-hydrological model was driven by output of the climate model ARPEGE under the A1B emission scenario over 30-year simulation periods. Our study indicated that with climate warming and increased atmospheric CO2, the main trends in water balance were a decrease in annual actual evapotranspiration (AET), a decrease in annual discharge and wetland extent, and a decrease in spring and summer of groundwater recharge and soil-water content. Not considering the effects of increased atmospheric CO2 in the agro-hydrological model led to overestimating discharge decrease and underestimating AET decrease and wetland extent. Climate change could influence N cycling by increasing soil N mineralisation, increasing soil denitrification in wetlands and upstream areas, and decreasing NO3-N load to streams. Since wetlands appear to be sensitive to climate change, improving modelling to better predict their responses is an important issue, especially to help plan sustainable management of these vulnerable areas