Since the beginning of the 21^st century, glaciers worldwide have been rapidly losing mass due to anthropogenic climate change. This mass loss generates natural hazards such as floods and landslides; and jeopardizes freshwater resources and energy supply in mountainous regions.

The role of groundwater in the water cycle of catchments fed by temperate glaciers is poorly characterized, especially the water fluxes occurring between the subglacial drainage system, the aquifers, and the surface flow network; as well as the glacial meltwater repartition in between these compartments. This is explained by the diversity of research fields and methods involved, the complexity of the hydrological system, the impossibility of directly observing the subglacial drainage system, and the fact that groundwater in glacial catchments has been studied very little to date, resulting in a lack of data on these aquifers.

In this thesis, I address these issues by collecting contemporary data in all compartments of the hydrosystem, so that co-occurring processes can be distinguished. I study four outlet glaciers of Vatnajökull ice cap, South-East of Iceland.

Two main aquifer formations are present in the region, separated by an aquitard of approximately 1 meter thickness, which may be discontinuous. The tills and glacio-fluvial deposits aquifer is unconfined, has a thickness of 2 to 54 m, and a hydraulic conductivity of 1e-6 to 1e-5 m/s. The basalt aquifer is unconfined when outcropping and confined when covered by the aquitard and the tills and glacio-fluvial deposits formation. It has a thickness of 200 to 400 meters, and a hydraulic conductivity of 1e-10 to 1e-5 m/s.
The rivers are braided or torrential and isolated, so it is not possible to measure surface water flow using classic gauging stations. I use passive seismic acquisition and develop two methods to calibrate the seismic records with a few point discharge measurements. Both methods provide accurate time series of absolute water flow rate, with a relative uncertainty of 5 to 15 %.

I observe subglacial channels under the ablation zone of the glaciers through dye-tracing experiments; with cross-sectional areas of 1 to 2 m2, and subglacial water velocities of 0.16 to 0.5 m/s. In 2023, the system was channelized from June to September. I observe a contemporary inefficient drainage system in early June and a solely inefficient drainage system in late September, with a subglacial flow rate of 0.19 m/s. These results will help understand and numerically model the water fluxes occurring between the aquifers and the subglacial drainage system.

I use glacier runoff data obtained by Liebsch et al., in prep through ablation modeling; and estimate forefield runoff through soil water balance computation. I compare the total runoff to the discharge time series acquired by passive seismic monitoring. The water balance computation at catchment scale indicates that 70 % of the water flowing beneath the glacial ablation zone infiltrates into the underlying aquifers during the melting season, and a smaller proportion beneath the glacier accumulation zone. In winter, exfiltration towards rivers or the subglacial drainage system occurs on a daily time scale, but the net balance in winter is still 60 % infiltration. Over a hydrological year, 50 % of the water available below the glacier infiltrates in the ground. The hydraulic head and groundwater temperature timeseries collected in a network of boreholes shows that upward vertical flow from the basaltic aquifer towards the tills and glacio-fluvial deposits aquifer is occurring in winter.

All these results help to improve the conceptual model of water flows in catchments fed by temperate glaciers, and the complementary data collected can contribute to the calibration and validation of a global numerical model of the area.

Lieu
École normale supérieure – PSL
24 rue Lhomond – aile Erasme
salle Claude Froidevaux – E314

Lien visio
https://visio.numerique.gouv.fr/wgf-eool-vua

Gwenn Flowers, Simon Fraser University – Reviewer
Clément Roques, Université de Neuchâtel – Reviewer
Francesca Pellicciotti, Institute of Science and Technology Austria – Examiner
Arnaud Burtin, Institut de Physique du Globe de Paris – Examiner
Sophie Violette, Sorbonne Université & Ecole Normale Supérieure – Thesis supervisor
Guðfinna Aðalgeirsdóttir, University of Iceland – Thesis supervisor
Olivier Gagliardini, Université Grenoble Alpes – Invited
Aude Vincent – Invited