Using water use models to project long-term trends of water supply and demand equilibriums under climate change: application to the French Loire River basin.
Authors: Debein Camille , Vermeil Victor , Lamouroux Raphaël , Monteil Céline , Hendrickx Frédéric , Zaoui Fabrice , Samie René 
Published: March 2024

The French Loire River basin (81,000 km&#178;) is characterized by a wide range of water uses (energy production, irrigation, drinking water supply, industrial processes, navigation, etc.). Recurring droughts and periods of low flow in the basin have emphasized the vulnerability of certain ecosystems and water uses in relation to the available resources. In addition, the prospect of global and local changes (climate change, changes in uses and territorial dynamics, etc.) added to evolutive environmental policies are expected to impact the availability of water resources in the upcoming years.Within this evolving context of resource availability, we propose a quantified projection of future changes in the water supply-demand balance within the Loire River basin for two future timeframes, 2035-2065 (mid-term) and 2070-2099 (long-term), relative to the current climate (1976-2005). To achieve this, a modeling framework encompassing catchment-scale representations of climate, natural resource distribution, and primary water uses (energy, irrigation, drinking water supply, and industry) has been developed. Spatial and temporal heterogeneities are accounted for with a semi-distributed hydrological model [1] (using sub-catchment meshes of nearly 100 km2) and a daily time-step. This framework builds upon previous hydrological studies [2] and enables the representation of impacts on resource availability resulting from both natural and anthropogenic forcing variables.Initially validated over the historical period (1976-2005) through comparison with national discharge monitoring networks and water use databases, this modeling chain was fed with data from four climate evolution trajectories taken from the Explore2 project that outline contrasting storylines of climate changes over the Loire basin. Simulation results reveal a decrease in water resources and an increase in global water demand, particularly in summer, correlating with increasing average air temperature and the relative reduction of precipitation. Evaluation of water stress indicators [3] suggests that tensions between water supply and demand will become increasingly frequent and more intense, particularly in summer.This work emphasizes the interest of coupling water use modeling with hydrological simulations and advocates for evaluating the impact of changes in the territory (such as socio-economic or land use dynamics) on the resource.Figure 1. Schematic representation of the modeling chain involved in quantifying the supply-demand balance.(A) (B)Figure 2. Spatial heterogeneity of the Blue Water Stress (A) and the Blue Water Scarcity (B) indicators [3] evaluated over the considered catchment area of the Loire River (august 2070-2099 for the &#8220;hot and humid&#8221; Explore2 climate trajectory, RCP 8.5).&#160;References:[1] Rouhier, L., Le Lay, M., Garavaglia, F., and Le Moine (2017). Impact of mesoscale spatial variability of climatic inputs and parameters on the hydrological response. Journal of Hydrology, 553, 13-25.[2] Samie, R., Monteil, C., Arama, Y., Bouscasse, H., and Sauquet, E. (2014). La prospective territoriale, un outil de r&#233;flexion sur la gestion de l&#8217;eau du bassin de la Durance en 2050. Hydrology in a Changing World: Environmental and Human Dimensions, 221.[3] Wang, Dan, Klaus Hubacek, Yuli Shan, Winnie Gerbens-Leenes, and Junguo Liu. (2021). "A Review of Water Stress and Water Footprint Accounting" Water 13, no. 2: 201.