Assessing water and energy fluxes in a regional hydrosystem: the Seine basin case study
Résumé
As water and energy resources is competitively used for drinking, recreation, irrigation, industrial and geo/thermal applications, the focus on climate change affecting the sustainable use increases. Multiple uses of water lead to high competition between different interest groups.
While it is well accepted that climate change and growing water needs affect the long-term sustainable water resources management, performing accurate simulations of water cycle and energy balance dynamics at regional scale remains a challenging task.
Traditional soil-vegetation-atmosphere-transfer (SVAT) used for numerical surface water and energy simulation do not treat the groundwater lower boundary in a fully process-based fashion. Conversely, groundwater models for saturated and unsaturated water flow, while addressing important features such as subsurface heterogeneity and stream-aquifer exchanges, often have overly simplified upper boundary conditions that ignore soil heating, vegetation processes on radiation fluxes, and root-zone uptake.
One of the first attempts of jointly modeling water and energy fluxes with a special focus on both surface and groundwater at the regional scale is demonstrated on the Seine basin ( 78,650 km² ), which overlays the main aquifer system of Europe.
The tool couples the SVAT model ORCHIDEE and the process-based hydrological–hydrogeological model CaWaQS. An original transport library based on the resolution of the diffusion/advection transport equation was written to simulate heat transfer in both 1D river networks, and pseudo-3D aquifer systems. In addition, an analytical solution is used to simulate heat transport through the aquitards and the streambed. Water and energy fluxes fractionated into fast surface run-off and slow infiltration towards the aquifer system are calculated by ORCHIDEE.
The model allows us to establish the first fully consistent water and energy budget over a pluriannual period of time, for which the hydrological variables are stationary. It also simulates temperature evolution in each aquifer and evaluates river thermal regulation by groundwater along the river network. Finally, climate change impacts on the Seine basin hydrological and thermal functioning are investigated using the calibrated model.