Shallow aquifer modeling by disaggregating regional scale systems to simple hillslopes under Boussinesq approximation
Résumé
Groundwater flow and transport models are primarily based on 3D processes accounting for multiple superposed
aquifers. While this approach is adapted to the widest variety of aquifers, it can be simplified in
some cases where flows do not extend other multiple layers. We propose a simplified approach based on the
decomposition of regions in watersheds and hillslopes, which can be deployed at the regional scale to assess
aquifer responses to global climate evolutions and natural resource evaluations. We propose that shallow aquifers
can be modeled by simple hillslope dominated free-surface flows, under Boussinesq approximation, with storage
limitation to account for possible return flow and subsurface excess overland flows.
Shallow groundwaters are sustaining rivers discharge, where it becomes a major contribution during dry
seasons. Though, little is known on aquifer properties and actual contribution to water cycle. Well networks are
either too localized, or too scarce with respect to the extent of the flow structure and the large heterogeneity of
aquifer systems to bring sufficient constrains.
One regular approach consists in analyzing river baseflow, and tracking the signature of groundwater contribution
within the watershed response. Though, the interpretation of these recessions is difficult, considering
that analytical solutions are available at hillslope scale, while analysis is performed at catchment scale. One
question arises on how the internal structure of the catchment, its geometry and natural heterogeneity affects the
interpretation of recessions.
Our strategy consists to perform the recession analysis on a hillslope-based model. Hillslopes can be aggregated
to describe the equivalent response at catchment scale, providing insights into how elementary hillslopes
contribute individually to sustain river flow. A last step consists in recovering aquifer properties from recessions.
We apply this approach to the Brittany region (France), in crystalline context, where aquifers systems are generally
shallow and unconfined. We performed the analysis on a range of watersheds (30km2 to 2000km2), which are
decomposed in hillslopes of 0.1km2 to 1km2. We show how heterogeneity controls the structure of the recession.
Such modeling and parameterization approach is valuable to infer watershed aquifer properties and supports
regional modeling to estimate the groundwater contribution to water cycle. As such, this approach will
provide a calibration tool for model application within the AquiFR project, a French national project whose goal
is to provide a global modeling tool of French aquifers based on segmented regional applications