• Energy Research

The main objective of this paper is to study the impacts of climate change on the hydrological cycle of the main French river basins, including the different uncertainties at stake. In particular, the relative importance of modeling uncertainty versus that of downscaling uncertainty is investigated. An ensemble of climate scenarios are statistically downscaled in order to force a hydrometeorological model over France. Then, the main changes in different variables of the hydrological cycle are studied. Despite large uncertainties linked to climate models, some robust signals already appear in the middle of the 21st century. In particular, a decrease in mean discharges in summer and fall, a decrease in soil moisture, and a decrease in snow cover, especially pronounced at the low and intermediate altitudes, are simulated. The low flows become more frequent but generally weak, and uncertain changes in the intensity of high flows are simulated. To evaluate downscaling uncertainties and assess the robustness of the results obtained with the statistical downscaling method, two other downscaling approaches are used. The first one is a dynamical downscaling methodology based on a variable resolution atmospheric model, with a quantile‐quantile bias correction of the model variables. The second approach is based on the so‐called anomaly method, that simply consists of perturbing present climate observations by the climatological change simulated by global climate models. After hydrological modeling, some discrepancies exist among the results from the different downscaling methods. However they remain limited and to a large extent smaller than climate model uncertainties, which raises important methodological considerations.

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.

This study aims at modelling the daily concentrations of nonylphenolic compounds such as 4-nonylphenol (4-NP), nonylphenol monoethoxylate (NP1EO) and nonylphenoxy acetic acid (NP1EC) within the Seine River downstream of Paris City for over a year, firstly in the present state (year 2010) and for years 2050 and 2100 in order to assess the consequences of global change on the fate of nonylphenolic compounds in the Seine river. Concentrations were first simulated for the year 2010 and compared to monthly measured values downstream of Paris. To achieve this goal, the hydrodynamic and biogeochemical model, ProSe, was updated to simulate the fate of 4-NP, NP1EO and NP1EC. The Seine upstream and Oise River (tributaries of the Seine River) concentrations are estimated according to concentrations-flow relationships. For Seine Aval wastewater treatment plant (SA-WWTP), the concentrations are considered constant and the median values of 11 campaigns are used. The biodegradation kinetics of 4-NP, NP1EO and NP1EC in the Seine River were deduced from the results of the companion paper. The Nash-Sutcliffe coefficient indicates a good efficiency to simulate the concentrations of 4-NP, NP1EC and NP1EO over an entire year. Eight scenarios were built to forecast the impacts of global warming (flow decrease), population growth (SA-WWTP flow increase) and optimisation of wastewater treatment (improvement of the quality of effluents) on annual concentrations of 4-NP, NP1EO and NP1EC at Meulan by 2050 and 2100. As a result, global warming and population growth may increase the concentrations of 4-NP, NP1EC and NP1EO, especially during low-flow conditions, while the optimisation of wastewater treatment is an efficient solution to balance the global change by reducing WWTP outflows.

Abstract. SURFEX is a new externalized land and ocean surface platform that describes the surface fluxes and the evolution of four types of surfaces: nature, town, inland water and ocean. It is mostly based on pre-existing, well-validated scientific models that are continuously improved. The motivation for the building of SURFEX is to use strictly identical scientific models in a high range of applications in order to mutualise the research and development efforts. SURFEX can be run in offline mode (0-D or 2-D runs) or in coupled mode (from mesoscale models to numerical weather prediction and climate models). An assimilation mode is included for numerical weather prediction and monitoring. In addition to momentum, heat and water fluxes, SURFEX is able to simulate fluxes of carbon dioxide, chemical species, continental aerosols, sea salt and snow particles. The main principles of the organisation of the surface are described first. Then, a survey is made of the scientific module (including the coupling strategy). Finally, the main applications of the code are summarised. The validation work undertaken shows that replacing the pre-existing surface models by SURFEX in these applications is usually associated with improved skill, as the numerous scientific developments contained in this community code are used to good advantage.

Although nowadays groundwater resources are mostly affected by increasing water abstraction, they will have toface the impact of climate change. Even in a temperate climate as the one in northern France these two driverswill have strong impact on the aquifers. Evolution of groundwater resource is most often studied based onevolution of the rainfall infiltration that is assumed to be the aquifer recharge, although river-aquifer and multilayeraquifer exchanges are also part of the recharge and can only be estimated using a hydrogeological model.Moreover, providing a change of the piezometric level is usually not found to be meaningful by stakeholder as a 1meter variation can have different impact depending on the hydrogeological context. Therefore, in this study, toprovide greater insight into the impact of groundwater, two new indicators are used: the evolution of the extensionof groundwater-fed wetlands and the frequency with which a crisis piezometric head is exceeded. The crisispiezometric levels are used locally by stakeholders and as soon as the observed piezometric heads fall below theselevels, actions are taken to preserve the water resource. For the present study, the local crisis piezometric heads were extended to the whole studied basins, the Seine andUpper Rhine Graben basins. The results show that the two basins under study are not affected in the same way by climate and groundwaterabstraction changes. The Seine multi-layer aquifers are projected to suffer from a substantial decrease in rainfall infiltration (about 20%), while the decrease is lower in the Upper Rhine Graben aquifer (about 6%).In 2050, the extension of groundwater-fed wetlands is expected to lose about 900m2/km2, while the crisispiezometric heads are expected to be exceeded about 40% and 26% of the time on average on the Seine and UpperRhine Graben aquifers, respectively. The reduction of the groundwater abstraction has a large impact locally but isnot efficient enough to overcome the impact of climate change, although the impact is more sensitive on the Upper Rhine Graben than on the Seine, and particularly on the extension of wetlands and low flow.Sensitivity to the parameters that drive the river-aquifer exchange was found to be second order compared to theuncertainty due to the climate model in the Upper Rhine Graben, although these parameters have a considerable impact on present-day estimation of the aquifer balance. The two indicators are found to be highly useful to discuss the impact of climate change on groundwater with stakeholders.In order to be able to extent such studies to the whole aquifers of France, and to have a survey of such impact as soon as new downscaled projections are available, the AquiFR project is bringing together hydrogeologicalapplications used by stakeholder in a common plateform that will be based at Météo-France