Description du sujet de thèse

In the context of global change, increasingly extreme weather events, and population growth, pressure on freshwater resources is mounting. There is an urgent need to rethink how water is shared (Pricope et al., 2025). On the one hand, we need to have a thorough understanding of the resource (both surface and groundwater) and, on the other hand, we need to have a thorough understanding of how it is used. New tools integrating these two dimensions are needed so that decision-makers and stakeholders in the territories can act at the right time to fairly and efficiently distribute this increasingly scarce freshwater. Only they have the keys to deciding priorities in terms of health, ecosystem protection, and economic issues within their territory.

The development of such tools must involve scientists, citizens, stakeholders, and political decision-makers in a process of co-construction, in order to maximize the possibility that the tools will be effective and adapted to the needs of the regions that wish to implement them. In this framework, this thesis focuses on the evolution of water resources from a quantitative perspective (river flows, groundwater volume and level, soil moisture) in metropolitan France, which requires particular attention to be paid to agricultural irrigation, as it accounts for 60% of annual freshwater consumption in France, and more than 80% in summer (Arambourou et al., 2025).

The first objective will be to develop a tool to simulate the temporal evolution of hydrological variables and their seasonal variations over the long term (trends) and on a territorial scale, taking into account the constraints imposed by climate change, changes in land use, and associated water withdrawals. This work will be based on an analysis of resource use (withdrawals and consumption by sector over time and space), which will also identify a number of watersheds of high importance for more in-depth forward thinking with stakeholders and decision-makers in these territories. The second objective of the thesis will be to develop plausible and contrasting scenarios for the joint evolution of water resources and agricultural uses in the selected territories, in collaboration with stakeholders, using the simulation tool developed to take into account supply-demand interactions under the influence of climate change.

From a methodological point of view, the integrated model of water resource and use will be based on the ORCHIDEE model, which describes land surfaces in the IPSL climate model. The ORCHIDEE model provides an accurate, high-resolution simulation of changes in the water balance in France since 1960, including changes in snow cover, based on climate, soil, and vegetation data (Huang et al., 2024). ORCHIDEE is one of the 10 hydrological models used to establish the hydro-climatic projections of the national Explore2 project (Sauquet et al., 2025). These projections are fed by a wide range of regionalized climate projections corresponding to different radiative forcing scenarios, and make it possible to characterize hydrological impacts and their uncertainties in a spatialized manner across mainland France and for different levels of warming. Finally, the ORCHIDEE model includes a biophysical description of irrigation (Arboleda-Obando et al., 2024), which has enabled the first study of the coupled evolution of water resources and irrigation during the 21st century in response to anthropogenic climate change (Arboleda-Obando et al., 2025).

To meet the first objective, this model will need to improve the groundwater resource simulation, to add seasonal inputs from artificial reservoirs and transfers between watersheds, and to incorporate non-agricultural withdrawals. The necessary data will be obtained from national and European inventories and remote sensing products (irrigated areas, water bodies, soil moisture, groundwater by gravimetry, etc.). We will verify that this tool better captures past hydrological trends in France, particularly in highly anthropized regions. Factorial simulations (deactivating anthropogenic pressures one by one) will then be set up to identify the main causes of these trends, using an attribution approach (Vicente-Serrano et al., 2019; 2019; Sebastian et al., 2019).

The target areas for scenario planning will be chosen for their current or future vulnerability to water shortages (e.g., the Pyrénées-Orientales, the Adour, Drôme, and Marais Poitevin river basins, or further north, those of the Seine and Moselle), and the interest of local stakeholders, ongoing collaborations, and the potential availability of observatories or in situ experiments. The scenarios co-constructed in these territories will enable factorial simulations to be extended to 2050 or 2100, in order to identify scenarios/regions where water resources are likely to be depleted, with negative impacts on the sustainability of water resources and socio-economic systems (tipping points), as opposed to scenarios/regions where sustainable compromises seem possible.

At the end of the study, the data and scenarios produced may be integrated into water resource management processes, as implemented within the PNACC (National Climate Change Adaptation Plan), among others. This work will contribute to WP3 of the Topic Center “Analysis and Modeling” of the ERIC eLTER “Support for the simulation of scenarios and storylines assessing the impacts of environmental and anthropogenic changes on ecosystem functioning and services,” thus benefiting from the state-of-the-art expertise of the European teams involved (FZJ and UFZ in Germany, UCan in Spain, INRAE and CNRS in France).

Bibliographic references
Arambourou, H., Ferrière, S., Belaud, G., Bouarfa, S. (2025). Quelle quantité d'eau utilisons-nous ? p31-49 in Comment partager l'eau en France ? À l'ère de l'anthropocène, édité par Sami Bouarfa, Marielle Montignoul, Thomas Pelte, Eric Sauquet, Quae, ISBN 978-2-7592-4098-2
Arboleda-Obando PF, Ducharne A, Yin Z, and Ciais P (2024). Validation of a new global irrigation scheme in the land surface model ORCHIDEE v2.2. Geoscientific Model Development, 17, 2141–2164, https://doi.org/10.5194/gmd-17-2141-2024.
Arboleda Obando PF, Ducharne A, Cheruy F, Ghattas J (2025). Joint evolution of irrigation, the water cycle and water resources under a strong climate change scenario from 1950 to 2100 in the IPSL-CM6. Earth Syst. Dynam. Discuss. [preprint], https://doi.org/10.5194/esd-2024-41, in revision.
Huang P, Ducharne A, Rinchiuso L, Polcher J, Baratgin L, Bastrikov V, Sauquet E (2024). Multi-objective calibration and evaluation of the ORCHIDEE land surface model over France at high resolution. Hydrology and Earth System Sciences, 28, 4455–4476, https://doi.org/10.5194/hess-28-4455-2024
Sauquet, E., Evin, G., Siauve, S., and 31 other authors incl. Ducharne, A. and Huang, P. (2025). A large transient multi-scenario multi-model ensemble of future streamflow and groundwater projections in France. Hydrology and Earth System Sciences Discuss., EGUsphere [preprint], https://doi.org/10.5194/egusphere-2025-1788
Pricope, N.G., Vicente-Serrano, S.M., Toreti, A. et al (2025). Increasing aridification calls for urgent global adaptive solutions and policy action. Nat Water 3, 512–515. https://doi.org/10.1038/s44221-025-00432-9

Contexte de travail

The PhD project is part of a partenship between the CNRS-INSU and the Ministry of Ecological Transition (MTE). The doctoral student may be called upon by this partner administration to share the content of their work, and raise awareness about their approach and results. It is expected that approximately 15% of the thesis time will be spent within this administration (MTE, La Défense site, Nanterre; conditions to be arranged).

The PhD project will be supervised by Agnès Ducharne (research director at the CNRS) and prepared within the GRNE Doctoral School (Geosciences, Natural Resources and Environment, ed398.sorbonne-universite.fr) and the research laboratory METIS (www.metis.upmc.fr), located at Sorbonne University, 4 place Jussieu, 75005, which will provide an office, computer, and computing resources. METIS is part of the IPSL (Institut Pierre Simon Laplace, www.ipsl.fr), which will be associated with thePhD project through the development of the ORCHIDEE model. Participation in conferences and publication costs will be covered by the supervisor's projects, in particular the PEPR TRACCS (Transforming Climate Modeling for Climate Services) project or, where applicable, by the partner administration.

Candidates must hold a Master's degree or equivalent in geosciences or environmental sciences, with solid skills in hydrology, agronomy, or climatology. The PhD project requires experience, or at least a strong interest, in modeling and quantitative methods; it will involve working in a Unix environment and developing Fortran code (ORCHIDEE model) and analysis scripts (in Python, R, or Matlab). The successful candidate must be fluent in French and English (spoken and written), have an affinity for translating research into action within the partner administration, and be committed to communicating their approach and results to public decision-makers, elected officials, and operational or political administrations.