Surface water and groundwater are closely linked. The latter interacts with the surface, on which it largely depends. In this way, groundwater is replenished on the one hand by natural diffuse recharge from rain and melting snow on the ground, and on the other hand by natural indirect recharge from localised seepage from rivers or lakes.
Whichever the natural recharge process involved, groundwater is highly dependent on climate.
- Firstly, in a direct way, through changes in the climate as a function of the water balance at the Earth's surface.
- But also indirectly, via the changes in groundwater abstraction required to meet the various water uses that have been affected by a warmer climate (irrigation in particular).
We can therefore distinguish between direct effects that are imposed on us, and indirect effects that result from our reaction to this new climate situation. The Explore2 research project recently analysed the impact of climate change on aquifer recharge in mainland France and came to a number of conclusions.
Increased evapotranspiration and more intense rainfall
In future climate conditions, the distribution of rainfall around the world is expected to change, with a decrease in low-intensity rainfall and an increase in the frequency of heavy rainfall, particularly in tropical regions, where more than half the world's population is expected to live by 2050. This phenomenon will be accompanied by longer and more frequent droughts.
In a warming world, evapotranspiration tends to increase overall, thus reducing the amount of ground surface water available for infiltration and run-off.
Recharge of aquifers increasingly uncertain
The risk of flooding by rising groundwater tables is associated with areas where recharge is increasing, while the risk of drought threatens sectors where recharge is decreasing, resulting in significant changes to the hydrological regimes of surface waters (rivers, lakes, wetlands, etc.).
Impacts on the quality of groundwater
We talk a lot about water volumes and recharge rates, but climate change is also altering the quality of groundwater:
- leaching of contaminants previously stored in the soil;
- increased concentrations of solutes such as chlorides, nitrates and arsenic in soils and surface groundwater, linked to an increase in evapotranspiration;
- risk of salinisation of coastal soils and aquifers.
Higher abstractions, an indirect effect
Climate change is likely to result in farmers drawing more water from aquifers to cope with the increase in evapotranspiration linked to global warming and the variability and decrease in soil moisture or water available at the surface.
In France, the "potential recharge" of aquifers is affected differently from one region to another
By cascading climate models with hydrological models, the researchers have simulated changes in the potential recharge rate of aquifers, according to two scenarios of changes in greenhouse gas emissions into the atmosphere (average or high emissions).
Potential recharge is defined as the proportion of effective precipitation likely to infiltrate from the surface and recharge the underlying aquifers, provided they have favourable hydrodynamic characteristics. It is qualified as potential because its arrival in aquifers cannot be known a priori and part of this flow may return to watercourses downstream from its infiltration zone.
According to this research, potential annual recharge could increase by between +10% and +30% in the north and north-east of France by the middle of the century. It would remain stable overall for the rest of the country – for both scenarios studied.
In addition, cumulative winter recharge would increase for almost all of France, with the exception of a southern strip and part of Brittany. In a scenario of high emissions, potential annual recharge would fall by between 10% and 30% in the south-west, south-east and Corsica by the end of the century.
The period at which potential recharge is at its maximum would also be brought forward by a month from spring to winter in the Alpine and Pyrenean ranges. With rising temperatures, precipitation will fall more in the form of rain, which infiltrates quickly, and less in the form of snow, which stores water until it melts.
In the case of the high emissions scenario, the maximum autumn recharge would be shifted by a month into winter around the Mediterranean. For the rest of the country, no significant changes were observed in the projections studied.
Reassuring but incomplete results
At first sight, these results seem reassuring, at least in the north of the country. However, they only take into account the direct impact of climate change on recharge, linked to the potential infiltration of rainfall.
It should be borne in mind that, for a large number of aquifers, infiltration of water from rivers accounts for a significant proportion of their recharge. The future evolution of river flows, which on an annual scale are expected to decrease in the southern half of France, must therefore also be taken into account.
In addition, indirect impacts must also be included in the analysis in order to develop future public policies aimed at ensuring sustainable management of aquifers on a national scale. One such is a possible increase in abstractions to cope with the increase in evapotranspiration and changes in land use to adapt to a changing climate.
