Field investigation of stream nitrate export controls in agricultural headwater catchments (Western France)

The objective of this work is to understand and to propose a conceptual model for the hydrological processes controlling N cycling and stream nitrate export (concentration, flux) throughout the hydrologic year in agricultural headwater catchments with shallow groundwater. Nitrate export by stream is controlled by hydrological processes related to water flow-paths, by nitrogen cycle processes (plant uptake, mineralization, nitrification and denitrification) or, most often, by the interaction of both. This interaction is expected to be different depending on the land use and the level of N saturation in soil. In agricultural areas,the nitrate export loadings in streams over time scales longer than the year can be related to the type of land use (rate of fertilizer application, organic matter mineralization rate).But at time scales of the year or shorter, the controls on nitrate export by streams are still poorly understood in agricultural catchments. In this context, a field experiment was carried out in stream and along hillslope groundwater of three agricultural catchments (Kervidy-Naizin, Kerrien and Kerbernez) in Brittany (Western France) over periods of 3 and 5 years. Hydrologic variables (stream discharge, water table depth, rainfall, PET) were recorded and daily to bi-weekly water samples were collected for analysis of major anions and geochemical parameters. In the all catchments, the nitrate flux depends on the amount of water that has flowed in the stream and not on the distribution of flow over the year. In winter, stream nitrate concentration and flux are mainly controlled by the concentration and discharge from the upland groundwater. In summer, stream concentrations occur at very low stream flow and depends only on bottom land groundwater and on in-stream processes. Processes such as hortonian flow or upward deep groundwater flow considered as non significant from a pure hydrologic point of view might have a significant role on summer stream nitrate concentration. The shift between winter and summer regimes depends on the spatial and temporal distribution of groundwater recharge. Unlike hydrologic models relevant for shallow groundwater catchment, nitrate transport modeling at the catchment scale over the hydrologic year should involve a representation of dominant hydrologic processes encountered in winter, such as shallow groundwater flow and hortonian runoff.