Groundwater controls on headwater stream dynamics

Headwater catchments, defined as the uppermost segments of drainage networks with intermittent and/or perennial third-order streams, are vital sources of freshwater and nutrients for downstream river basins. Despite their critical role in sustaining natural ecosystems and supporting human services, these systems remain poorly understood and are often referred to as 'aqua incognita¹.' A key challenge lies in unraveling the hidden groundwater processes that contribute to storage-discharge dynamics. Recent advances in both in-situ and remote monitoring, combined with innovative modeling techniques, now offer opportunities to capture the complex interactions between surface and subsurface processes across diverse climatic, topographic, and geological contexts.

In this presentation, we will present recent findings from field investigations conducted in headwater observatories, complemented by numerical modeling experiments designed to evaluate the controls of key geomorphic factors on groundwater-surface water interactions. The presentation will explore how landforms, lithologies, subsurface stress, and faults shape hydrological behaviors, including stream baseflow recession, groundwater seepage distribution, flow intermittency, and water residence times. Additionally, we will highlight advances in numerical modeling techniques, particularly through the HydroModPy community modelling platform², which enhance the representation and calibration of groundwater processes in catchment-scale hydrological models. Through the application of these models on pilot sites, we will illustrate how subsurface heterogeneity influences the predictions of water availability under future climate change scenarios, emphasizing the importance of integrating hydrogeological insights for supporting resilient water resource management.

¹ Bishop, K., Buffam, I., Erlandsson, M., Fölster, J., Laudon, H., Seibert, J., Temnerud, J., 2008. Aqua Incognita: the unknown headwaters. Hydrological Processes 22, 1239–1242. https://doi.org/10.1002/hyp.7049

² Gauvain, A., Abhervé, R., Coche, A., Le Mesnil, M., Roques, C., Bouchez, C., Marçais, J., Leray, S., Marti, E., Figueroa, R., Bresciani, E., Vautier, C., Boivin, B., Sallou, J., Bourcier, J., Combemale, B., Brunner, P., Longuevergne, L., Aquilina, L., and de Dreuzy, J.-R.: HydroModPy: A Python toolbox for deploying catchment-scale shallow groundwater models , EGUsphere [preprint], https://doi.org/10.5194/egusphere-2024-3962, 2025.