How Climate Change Disrupts Groundwater Recharge

groundwater is in close connection. The latter interact with the surface, on which they largely depend. Thus, the underground water tables are renewed on the one hand by the diffuse natural recharging brought by the rain and the melting of snow on the floors, on the other hand by the indirect natural recharge that bring in the infiltrations located from rivers or lakes.

Whatever the natural charging process concerned, groundwater is therefore strongly dependent on the climate.

• First directly, through the changes of the latter, depending on the water balance on the surface of the earth.
• But also indirectly, via changes in the groundwater samples necessary to respond to the different uses of water impacted by a warmer climate (irrigation in particular).

There are thus direct effects imposed on us, and indirect effects which result from our reaction to this new climate situation. The explore2 research project recently analyzed the consequences of climate change on the recharge of aquifers in mainland France and has enabled a number of observations.

Increased evapotranspiration and more intense precipitation

The climate and plant cover largely control precipitation and evapotranspiration, while the soil and the underlying geology determine if the excess water can infiltrate an underlying aquifer or if it will rather run towards a river.

In detail, the future evolution of the recharging depends primarily on the modification of the quantity and the rainy regime.

However in the future climate, we expect a change in the distribution of precipitation worldwide, with a decrease in low intensity precipitation and an increase in the frequency of strong precipitation, in particular in tropical regions, where more than half of the world's population should live by 2050. Phenomenon which will be accompanied by longer and more frequent droughts.

In a world that heats up, evapotranspiration has generally tend to increase, which reduces the amount of water from the ground available for infiltration and runoff.

An increasingly uncertain aquifers recharge

Less frequent but more abundant precipitation could improve the share of groundwater recharging in many semi-arid surrounding environments. But when the intensity of the rains exceeds the capacity of the soil to infiltration, there is an increase in runoff and the flows of the rivers. These contrasting changes in charging rates cause more or less rapid changes in water levels within water tables, depending on the speed of groundwater.

Risks of flooding by raising a tablecloth are associated with the areas where recharging increases while risk of drought threatens the sectors where recharge decreases, with the consequence of significant modifications of the hydrological diets of surface water (rivers, lakes wetlands, etc.). For example, river low water flows decrease in hydrographic basins where the level of tablecloths decreases in response to a decrease in recharge.

In addition to precipitation, the melting of glaciers and snow in the mountains often also significantly contributes to the recharge of mountainous aquifers. Its future increase in the effect of warming is likely to impact the regime of this recharge in a way that it is still difficult to analyze.

The decline in glaciers initially increases the production of cast iron water, until reaching a maximum, known as “water peak”, before decreasing as the glaciers continue to retreat. Today, about half of the world's glacial hydrographic pools would have exceeded the peak of water. Consequently, a decrease in the recharge of mountain aquifers is now expected in an increasing number of glacial basins.

Impacts on the quality of groundwater

We talk a lot about volumes of water and recharge rate, but climate change also alters the quality of groundwater. The more intense rain episodes and the resulting recharge are likely to mobilize contaminants by leaching which were previously stored in the soil.

On the other hand, warming can lead to the increase in solute concentrations such as chlorides, nitrates or arsenic in superficial soils and water, in connection with an increase in evapotranspiration, and a decrease in infiltration and therefore a lower dilution.

Finally, the increase in groundwater temperature, induced by global warming and locally by urban heat islands, modifies the solubility and concentration in the water of certain contaminants. These impacts are less often observed, but very real, especially around the Mediterranean.

In addition, the increase in sea level induces a risk of soil salinization and coastal aquifers worldwide. This saline intrusion depends on many factors such as coastal geology, topography and especially water levels in tablecloths. It can be particularly severe in low areas such as deltas, islands or even atolls where there are freshwater lenses particularly sensitive to climate change.

So much for direct impacts. But scientists believe that the impact of climate change on groundwater can be more important through indirect effects, linked to the effects of climate change.

The increase in samples, an indirect effect

Climate change may indeed cause an increase in levies in aquifers by farmers to cope with the increase in evapotranspiration linked to warming and variability and decrease in humidity in soils or water available on the surface.

The increase in pumping, for irrigation in particular, induces a chronic drop in water levels which threatens the sustainable management of tablecloths, water flows towards neighboring hydrosystems or surface ecosystems depending on groundwater (wetlands).

This increased pressure is likely to considerably modify the water cycle, with strong contrasts between:

• The exhaustion of groundwater in regions where irrigation is mainly powered by groundwater;
• the rise in the level of the tablecloths resulting from the recharging by the excess irrigation flows fueled by surface water and which can lead to a conflict and salinization of the soil;
• And the changes in local climates resulting from the increased evapotranspiration of irrigated land.

Consequences on “potential recharge” in France

The impact of climate change on the recharge rate of aquifers in mainland France was recently analyzed by the scientific community within the framework of the Explore2 project.

Thanks to the cascade of climatic models with hydrological models, the researchers simulated changes in the potential recharge rate of the tablecloths, according to two evolution scenarios of greenhouse gas emissions in the atmosphere (medium or strong emissions).

Potential recharge is defined as the share of effective precipitation likely to infiltrate from the surface and to recharge underlying aquifers, insofar as they have the favorable hydrodynamic characteristics. It is qualified as potential because its arrival in aquifers cannot be known a priori and part of this flow can return to rivers, downstream from its infiltration zone.

Regions affected differently

According to this work, the potential annual recharge could increase in the middle of the century between + 10 % and + 30 % in the north and northeast of France. It would remain generally stable on the rest of the country – for the two scenarios studied.

In addition, the accumulation of charging during the winter would increase for almost all of France, apart from a southern band and part of Brittany. In a scenario of strong emissions, the potential annual recharge would drop to the South West, the Southeast and Corsica at the end of the century, from -10 to -30 %.

The period at which potential recharging is at its maximum would also be advanced by a month from spring to winter on Alpine and Pyrenean chains. Due to the rise in temperatures, precipitation would indeed fall more in the form of rain, which infiltrates quickly, and less in the form of snow, which stores the water until the cast iron period.

In the case of the scenario of strong emissions, the maximum of fall recharging would be offset for a month around winter around the Mediterranean. For the rest of the country, no significant modification has been observed in the projections studied.

Reassuring but incomplete results

These results seem reassuring at first glance, at least in the north of the country. However, they only take into account the direct impacts of climate change on recharge, linked to the potential infiltration of precipitation.

However, for a large number of aquifers, the infiltration of water from rivers is an important part of the recharge. The future development of the flow rates of watercourses, which on a annual scale, should decrease in the southern half of the French hexagonal territory, must therefore also be taken into account.

In addition, indirect impacts must also be integrated into the analysis to develop future public policies aimed at ensuring sustainable management of aquifers across our country: as a possible increase in samples to cope with the increase in evapotranspiration and changes in land use to adapt to the changing climate.