A pan-European map of shallow aquifer transmissivity in crystalline headwater catchments inferred from wetland and stream networks

Groundwater systems in headwater catchments are poorly represented at continental scales due to model resolution constraints and limited observations available to characterize the wide diversity of catchments. Yet, low-order headwater streams accounting for a major fraction of the global river network. This is particularly true in upland crystalline bedrock regions with dense drainage networks, where the lithology has long been considered impermeable, without aquifers, and thus has received limited hydrogeological attention.

We present a new continental-scale assessment of effective transmissivity for 3,333 European crystalline headwater catchments (median ≈35 km²), underlain by unconfined, shallow hard-rock aquifers where subsurface-surface interactions strongly shape hydrological connectivity. Catchments including dams, glaciers, and extensive permafrost were excluded.

The methodology represents lateral hillslope groundwater flow within shallow subsurface systems, capturing the spatial patterns of saturated areas at the catchment scale. This framework of physically based groundwater flow models enables steady-state simulation of perennial surface water networks (springs, streams, wetlands), whose length and structure are highly sensitive to shallow aquifer transmissivity (Abhervé et al., 2023). Transmissivity was inferred through optimization of simulated seepage areas against observed wetland and stream networks, using constant recharge estimates from an independent land surface model and assuming dominant superficial subsurface flow in the upper 50 m. Across all calibrated models, the simulated networks closely replicate the available European-scale extended wetland ecosystem layer and stream network from the EU-Hydro database.

Estimated transmissivity ranges from 10⁻⁸ to 10⁻² m² s⁻¹ (mean ≈10⁻⁴ m² s⁻¹), with pronounced spatial variability across geological provinces, massifs, or regions sharing similar tectonic framework legacies. The broad transmissivity range demonstrates the method’s sensitivity and its ability to resolve catchment-scale effective hydraulic properties across diverse climatic, topographic, and geological contexts. Values are consistent with textbook estimates for the studied lithologies and with hydraulic test data (pumping and slug tests) from regional or global datasets. Both measurements and estimates follow a log-normal distribution. Hydraulic conductivity was also derived from transmissivity using independent aquifer thickness datasets, including global depth-to-bedrock and regolith thickness maps.

Our results provide the first EUropean crystalline bedRock hydrogeological HEADwater map of transmissivitY (EURHEADY), explicitly accounting for groundwater flows at the catchment scale. All calibrated simulations are provided as a georeferenced dataset, complemented by physiographic, climatic, hydrologic, pedologic, geologic, hydrogeologic, and anthropogenic attributes. This approach addresses a critical gap in estimating hydrogeological properties, a long-standing challenge for the critical zone community, and opens new opportunities for large-scale hydro(geo)logical modeling with improved representation of groundwater contributions.

Reference:
Abhervé, R., Roques, C., Gauvain, A., Longuevergne, L., Louaisil, S., Aquilina, L., & de Dreuzy, J. (2023). Calibration of groundwater seepage against the spatial distribution of the stream network to assess catchment-scale hydraulic properties. Hydrology and Earth System Sciences, 27(17), 3221–3239. https://doi.org/10.5194/hess-27-3221-2023