Integrating a New Groundwater Module into ORCHIDEE: Regional Effects of Stream Meandering on Groundwater Recharge

As climate change exacerbates the frequency and intensity of hydrological extremes, the restoration of natural river dynamics—such as stream meandering—has emerged as a promising nature-based solution (NBS) to enhance water retention, reduce flood risks, and sustain low flows. While local experiments demonstrate the benefits of meandering for ecosystem resilience, their large-scale impacts on groundwater recharge and regional hydrology remain poorly understood.Within the European NBRACER project, we explore the potential of upscaling stream meandering across France using the IPSL land surface model ORCHIDEE. However, the model’s initial groundwater representation lackscritical processes, including horizontal flow between grid cells and feedbacks between groundwater, soil moisture, and rivers. To address these limitations, we implemented an enhanced groundwater scheme [1] incorporating lateral exchanges and retroactive river-aquifer coupling. Using a regional configuration over continental France and the high-resolution hydrogeological dataset BDLISA, we simulate the hydrological effects of increased river length on groundwater recharge and low flows.This study highlights the technical challenges of integrating subgrid-scale processes in large-scale simulations and provides insights into the scalabilityof stream meandering as an NBS. Our findings aim to inform water management strategies and climate adaptation policies by assessing the broaderhydrological impacts of river restoration.[1] Vergnes, J.-P., Decharme, B., Alkama, R., Martin, E., Habets, F., & Douville, H. (2012). A simple groundwater scheme for hydrological and climate applications: Description and offline evaluation over France. Journal of Hydrometeorology, 13(4), 1149-1171. https://doi.org/10.1175/JHM-D-11-0105.