Improvement of the solute transfer in a conceptual unsaturated zone scheme : a case study of the Seine River Basin

International audienceFor 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. Applied to the Seine river basin, this model represents accurately the temporal evolution of average nitrate concentrations in the aquifer, but with large local errors. We propose an improvement of the simple unsaturated zone 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 a comparison with NonsatSW and Metis. In an ideal case, NonsatVG generates a solute transfer and a dispersion closer to that of Metis than 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 link between the water table depth 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 unsaturated zone, NonsatVG is also as valid as NonsatSW in the modelling of water transfer to the saturated zone. Finally, an application on the Seine basin show that solute transfer velocities are lower with NonsatVG than with NonsatSW, but in better agreement with literature

Assessing the water balance of the Upper Rhine Graben hydrosystem

International audienceThe Upper Rhine alluvial aquifer is an important transboundary water resource. However, as in many alluvial systems, the aquifer inflows and outflows are not precisely known because of the difficulty of estimating the river infiltration flux and the boundary subsurface flow. To provide a thorough representation of the aquifer system, a coupled surface-subsurface model was applied to the whole aquifer basin, and several parameter sets were tested to investigate the uncertainty due to poorly known parameters (e.g. aquifer transmissivity computed by an inverse model, river bed characteristics). Twelve simulations were run and analyzed using standard statistical criteria and also a more advanced statistical method, the Karhunen Loève transform (KLT). This analysis showed that, although the model performed reasonably well, some piezometric level underestimations persisted in the south of the basin. An accurate representation of the aquifer behaviour would require river infiltration and the functioning of irrigation canals in the Hardt area to be taken into account. It also appeared that increasing the maximum river infiltration flow deteriorated the quality of the results. River infiltration to the aquifer was estimated to represent about 80% of the aquifer inflows with a mean annual value around 115 ± 16.5 m3/s, thus with an uncertainty of 14%. This quantity is larger than estimated in previous studies but is in agreement with some results obtained during low water periods. This important conclusion highlights the vulnerability of the Upper Rhine Graben aquifer to pollution from the rivers and to climate change since it is highly probable that the rivers' regimes will be affected by reduced snow cover on the neighbouring mountain ranges

The SAFRAN-ISBA-MODCOU hydrometeorological model applied over France

An edited version of this paper was published by AGU. Copyright (2008) American Geophysical UnionThe hydrometeorological model SIM consists in a meterological analysis system (SAFRAN), a land surface model (ISBA) and a hydrogeological model (MODCOU). It generates atmospheric forcing at an hourly time step, and it computes water and surface energy budgets, the river ow at more than 900 rivergauging stations, and the level of several aquifers. SIM was extended over all of France in order to have a homogeneous nation-wide monitoring of the water resources: it can therefore be used to forecast flood risk and to monitor drought risk over the entire nation. The hydrometeorologival model was applied over a 10-year period from 1995 to 2005. In this paper the databases used by the SIM model are presented, then the 10-year simulation is assessed by using the observations of daily stream-flow, piezometric head, and snow depth. This assessment shows that SIM is able to reproduce the spatial and temporal variabilities of the water fluxes. The efficiency is above 0.55 (reasonable results) for 66 % of the simulated rivergages, and above 0.65 (rather good results) for 36 % of them. However, the SIM system produces worse results during the driest years, which is more likely due to the fact that only few aquifers are simulated explicitly. The annual evolution of the snow depth is well reproduced, with a square correlation coeficient around 0.9 over the large altitude range in the domain. The stream ow observations were used to estimate the overall error of the simulated latent heat ux, which was estimated to be less than 4 %

Modélisation de la pollution nitrique des aquifères bas-normands - Simulation de l'impact de changements de pratiques culturales sur la qualité des eaux souterraines

Nombre de pages : 131Rapport d'étude techniqu

Évolutions constatées et prévisibles des principales composantes du climat ayant un effet sur l'agriculture avec un focus sur l'hydrologie

International audienceL'augmentation de la concentration atmosphérique en gaz à effet de serre a des conséquences importantes sur le climat y compris pour des régions tempérées comme la France. Après quelques rappels sur les processus et les méthodes utilisées pour projeter les impacts, l'article se focalise sur l'évolution de la ressource en eau en France. On projette une diminution marquée des débits d'étiage et de la ressource en eau souterraine, ainsi qu'une occurrence accrue des sécheresses édaphiques. Cette évolution sur la ressource en eau sera une contrainte pour l'évolution et l'adaptation de l'agriculture, qui subit par ailleurs des modifications importantes de la phénologie et une amplification des stress thermique

Downscaling climate models for hydrology: Pitfalls and needs?

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