Regional scale studies

People involved:Philippe DAVY (CNRS), Jean DE BREMOND D'ARS (CNRS), Tanguy LE BORGNE (UR1), Pascal GODERNIAUX (UR1), Etienne BRESCIANI (UR1), Clément ROQUES (UR1), Laurent Longuevergne (CNRS), Olivier Bour (UR1)

In order to obtain a regional assessment of Brittany region groundwater resources and their potential sensitivity to climate change, a series of investigations were undertaken on different sites at regional scale (see map below). In particular, the Saint Brice en Coglès site corresponds to a deep crystalline aquifer situated at about 55km north east of Rennes. It is located in a similar geological setting as the Ploemeur site but in a different geographic and climatic context. Thus, it allows obtaining complementary data for assessing recharge mechanisms and climate change effect,  and it provides a better representativeness of hydrogeology at regional scale.  These data will be included into the modeling approach to characterize and quantify, at the regional scale, groundwater circulations in deep and shallow aquifers, in function of recharge conditions, topography, and surface - subsurface interactions.

Figure : sites were investigations related to the CLIMAWAT project have been undertaken by CNRS and University of Rennes 1.

 

The Brittany region has created a working group focusing on minimum river streamflow ensuring sufficient water quality and quantity for biological life, with a specific focus on crystalline context. This working group is called CRESEB (Centre de Ressources et d'Expertise Scientifique sur l'Eau de Bretagne, http://www.creseb.fr/) and gathers stakeholders, water management associations, state experts and researchers. CNRS is supporting the working group with the estimation of mass balance at watershed scale and river discharge considering both impacts of water management and climate change to water quality and ecological/biological preservation issues.

Ecological risks are maximal when river discharge is minimal, i.e. during dry periods, when river discharge is reduced to aquifer discharge, or baseflow. Knowledge of aquifer characteristics is here of prior importance.

The figure shows the number of days a year when streamflow is 10 times smaller than river module, a critical stage. It is compared to aquifer response time, which underlines how long an aquifer might release water with no new water influx (precipitation).

Physically, it is directly related to aquifer size, porosity and transmissivity. In an homogeneous context, aquifer response time is essentially controlled by its size, because of the ability of large watershed to catch deep water circulation, on contrary to smaller watersheds. In Brittany, small watersheds (Isole river in Scaër, 97 km²) might have response time around 40 days, ensuring sufficient water storage and release during dry periods, similar to large watersheds (Laita river in Quimperlé, 832 km²). This underlines the direct impact of the highly heterogeneous hydrogeological context. Local discharge of deep groundwater system might also be suspected.

Three basins have been selected for pilot studies, ranging from natural (Odet watershed), lightly anthropized (Ellé, Isole Laïta watershed) to heavily antrhopized (Rance watershed)

Modelling the impact of climate change on groundwater resources

People involved:Pascal GODERNIAUX (UR1), Philippe DAVY (CNRS), Jean DE BREMOND D'ARS (CNRS), Tanguy LE BORGNE (UR1), Etienne BRESCIANI (UR1), Jean-Raynald de Dreuzy (CNRS)

Published articles:

Goderniaux, P., P. Davy, E. Bresciani, J.-R. de Dreuzy1, T. Le Borgne, Partitioning a regional groundwater flow system into shallow local and deep regional flow compartments, Water Resour. Res., VOL. 49, 1–13, doi:10.1002/wrcr.20186, 2013

The distribution of groundwater fluxes in aquifers is strongly influenced by topography, and organized between hillslope and regional scales. In order to assess the impact of climate change on groundwater resources, we investigated the compartmentalization of aquifers at the regional scale, and the partitioning of recharge between shallow/local and deep/regional groundwater transfers (Goderniaux et al., in preparation). A finite-difference flow model was implemented and the flow structure was analyzed as a function of recharge (from 20 to 500 mm/yr), at the regional-scale (1400 km²), in 3-dimensions, and accounting for variable groundwater discharge zones; aspects which are usually not considered simultaneously in previous studies. The model allows visualizing 3D circulations, as those provided by Tothian models in 2D, and shows local and regional transfers, with 3D effects. The PDF of transit times clearly show two different parts, interpreted using a two-compartment model, and related to regional groundwater transfers and local groundwater transfers. The importance of each process is strongly influenced by the total recharge rate and the spatial distribution of discharge zones, which control the length of groundwater pathways. Results show that, while the absolute recharge rate feeding the 'regional' compartment decreases with total recharge, the proportion of total recharge feeding this compartment increase. The volume associated to the regional compartment is calculated from the exponential part of the two-compartment model, and is nearly insensitive to the total recharge fluctuations. This methodology constitutes an efficient methodology to  segregate and quantify local and regional fluxes .

Figure.View of the modeled area at the scale of the 'Brittany' region

Figure.Evolution of groundwater levels as a function of recharge

 

Figure. Approximated volumes corresponding to the different compartments, as a function of the total recharge flux.

Figure. Probability Density Functions of pathway length and travel times for different recharges.