Abstract. The repetition of droughts in France has led to a growing demand for irrigation water and consequently to an increase in requests for the construction of small farm dams. Although such dams are small, their accumulation in a basin affects the river flows. In order to gain more insight into their impact, a small farm dam model was built and connected to a hydrometeorological model. Several scenarios with different volume capacity, filling catchment size and filling period were tested for such dams. The results were analysed in a small basin in western France, where the pressure for building such dams is high, and then extended to the whole country. It was found that, due to the hydrometeorological conditions (mainly precipitation), the development of small farm dams in north-western France would lead to larger impacts on the riverflows and to less efficient filling of the small farm dams than in other regions of France. Moreover, such behaviour is projected to worsen in a context of climate change, despite the uncertainty on the evolution of precipitation.

Abstract. The repetition of droughts in France has led to a growing demand for irrigation water and consequently to an increase in requests for the construction of small farm dams. Although such dams are small, their accumulation in a basin affects river flows, because the water collected in these small farm dams is used for irrigation and thus does not contribute to river flow. In order to gain more insight into their impact on the annual and monthly discharges, especially during dry years, a small farm dam model was built and connected to a hydrometeorological model. Several scenarios with different volume capacities, filling catchment sizes and filling periods were tested for such dams. The results were analysed in a small basin in western France, where the pressure for building such dams is high, and then extended to the entire country. It was found that, due to the hydrometeorological conditions (mainly low precipitation compared to other regions in France), the development of small farm dams in north-western France would result in greater decreases in river flows and less efficient filling of small farm dams than in other regions. Therefore, such dams might not be as efficient as expected in supplying water to farmers when needed. Moreover, the ability to fill small farm dams is projected to decrease in a context of climate change, despite the uncertainty on the evolution of precipitation, thus worsening the situation.

Abstract:For predicting the evolution of solute concentrations in groundwater and testing the impact of remediation policies, a coupling between the agronomical model STICS and the hydrogeological model MODCOU was implemented. When applied to the Seine River basin, this model accurately represents the temporal evolution of average nitrate concentrations in the aquifer, but with large local errors.We propose an improvement to the simple unsaturated zone (UZ) scheme NonsatSW used in STICS-MODCOU. The modifications are based on a comparison with the mechanistic model Metis considered as a reference as it solves Richards' equation. A more realistic saturation profile and a varying percolation rate are integrated in NonsatSW. This new model, named NonsatVG, is assessed by comparing it with NonsatSW and Metis. In an ideal case, NonsatVG generates a solute transfer and a dispersion closer to that of Metis than of NonsatSW. In real cases, without additional calibration, NonsatVG and Metis simulate better the average transfer velocities of the observed nitrate profiles.Furthermore, modifications in NonsatVG give a direct relationship between the depth of the water table and the saturation profile. We obtain, therefore, as in Metis, an evolution of the solute transfer velocity depending on the piezometric level. These dynamics are not simulated in NonsatSW.Despite a modified water transfer through the UZ, NonsatVG is also as valid as NonsatSW in the modelling of water transfer to the saturated zone.Finally, an application to the Seine basin shows that solute transfer velocities are lower with NonsatVG than with NonsatSW, but are in better agreement with literature.