The water that circulates under our feet, deep in the interstices and fractures of the rock, is invisible. And yet, it is thanks to this underground water (called "water table") that during the dry summers, the rivers flow and the marshes do not dry up. Remember that groundwater ensures 65% of our drinking water supply.
If the month of May was particularly cool and humid, with abundant rains, will these rains make it possible to recharge the groundwater tables? We are talking here about the water tables that are commonly called "groundwater", that is to say those that are just under our feet, unlike those called "captive", deep and covered with a thick layer of water. clay (as in the Aquitaine Basin).
Well, not really: at the end of May, the groundwater levels are in fact mostly down compared to April. The water infiltrated into the soil following the May rains will have mainly served to moisten the soil, to the benefit of the vegetation.
However, these rainfall inputs should make it possible to postpone the irrigation campaigns and to alleviate, for the next few weeks, the pressure exerted by agricultural withdrawals.
According to the National Bank of Quantitative Water Abstractions, irrigation represents 15 to 23% of pumped volumes in groundwater, i.e. between 0,8 and 1,3 billion cubic meters per year for the period 2012-2018; the rest of the withdrawals concern drinking water (61 to 67%) and industrial uses (16 to 18%).
It will be understood that the volume of water pumped varies according to the crop's water requirement and therefore depends on the dryness of the soil.
Three dry and hot months on the horizon
Of course, the May rains did have an impact locally, on heavily watered areas, in Auvergne-Rhône-Alpes and Provence-Côte-d'Azur for example, and only on so-called “reactive” water tables. that is, where underground flows are rapid (such as alluvium from the Rhône, karstified limestones from Jura and Provence).
However, these increases are occasional and the levels stabilize, or even fall again, as soon as the rainfall is no longer sufficient to compensate for the emptying.
In areas with less water and more “inertial” water tables - where groundwater flows slowly (slow refilling and emptying) - it will be necessary to wait several days to weeks to find out whether the water has managed to infiltrate into it. depth. The trends will depend on local rainfall accumulations, evapotranspiration and water demands. Thus, certain reactive aquifers, such as the basement aquifers of the Massif Central, observe a stabilization of levels.
Knowing that the more inertial and deep the sheet, the weaker the impact will be. It will probably even be non-existent on the very inertial chalk slicks and tertiary formations of the Paris Basin.
Finally, it should be remembered that Météo France has announced that June, July and August 2021 would be hotter and drier than seasonal norms. In this context, the Bureau of Geological and Mining Research (BRGM) has made forecasts on groundwater tables for the summer of 2021. The forecast turns out to be quite pessimistic, with probably low levels for this summer; we even fear a very strong risk of drought in the East - from Alsace to Provence.
Only the inertial aquifers in the center of the Paris Basin and the Artois, as well as some aquifers in the Aquitaine Basin, will be spared.
Understanding the path of water
To better understand these complex phenomena - and in particular the reasons why heavy rains cannot always "recharge" groundwater tables - it is necessary to know the path of the water, from the surface to the water table.
In France, only 20 to 23% of the average annual precipitation manages to infiltrate in depth. This groundwater recharge takes place mainly in autumn and winter. In spring and summer, as we will see, all the conditions are in place to trap rainwater, prevent it from infiltrating deeply to recharge the water table.
The role of sun and runoff
Let's start with what is happening on the surface: the rain has just left its cloud that it can be intercepted and evaporated back to the atmosphere. The sunshine and the temperature of the air indeed act on the water fallen to the surface - leaves, roofs, grounds, ponds, sea… - and that up to several centimeters in the ground.
Once on the ground, part of the water will run off towards the watercourses; another part will seep into the ground. The choice of the type of flow is determined by the slope of the land, by the permeability and by the water saturation of the surface layer of the soil.
These last two parameters are governed by the texture (clay, silt, sand) and by the structure (organization of the grains) of the soil. The waters infiltrate if they have the possibility of stagnating and if there are connected and free spaces.
Un spatial indicator was designed by the BRGM to assess the soil's ability to run off or infiltrate surface water. This approach makes it possible to translate the part of water that infiltrates and runs off.
The most infiltrating soils concern the loamy soils of northern France and the soils developed on limestone rocks (Lorraine, Côte-des-Bars, Jura, Berry, Bassin de la Sarthe, Brenne, Poitou, Charentes, Périgord, Causses du Quercy, Grands Causse, Cévennes border, Provence).
Conversely, the most runny sectors are made up of marls, peat or clay with low permeability, steeply sloping land in the Alps and the Pyrenees and soils saturated with water located in valleys - the Alsace plain and large rivers, for example - and in wetlands - Camargues and Atlantic marshes.
Several elements, such as the intensity and quantity of rainfall, plant cover, biological activity and developments linked to human activities, can affect the soil's capacity to absorb water.
Summer thunderstorms, for example, are not conducive to effective infiltration. Heavy rains first of all impact the texture of the soils: they break up the soil on the surface and create an impermeable surface layer, called the “battance crust”, which promotes runoff.
Then, in the event of heavy rains, the soil is no longer able to absorb all the water, the pores being saturated. It should be noted that the plant cover makes it possible to attenuate the intensity of the rains, by creating a barrier to runoff water and maintaining a good structure of the soil.
Finally, it should be remembered that human developments have intensified soil waterproofing, creating new “water barriers”: asphalt roads, urbanization, deforestation or compaction of agricultural soils.
Actions are currently being implemented to allow rainwater to find its natural path. More and more metropolises are implementing a policy of infiltrating rainwater into the plot (as in Rennes or Paris). Farmers are also modifying their practices so as not to leave the soil bare (winter crops), to decompact the soil and to block runoff using hedges and grass strips.
Soil and plant retention
Once the surface layer is crossed, the water circulates in the pores of the soil and encounters new obstacles.
Obviously, soil permeability and water saturation always play an important role in the circulation of water in the deeper layers of the soil.
Soil particles exert capillary forces capable of retaining water. This is trapped in the form of moisture and its mobility becomes very reduced. The retention capacity of a soil depends on its texture and its porosity. Clays and soils rich in organic matter, as well as weakly porous soils, retain water more strongly.
Part of this immobilized water can be returned to the plants. This water is drawn by the roots and directed to the stems and leaves.
It is then evaporated by transpiration into the atmosphere. Evapotranspiration takes up almost all of the infiltrated water when vegetation is active, usually from April to October-November.
The labyrinth of gravity flow
It will be understood that the soil must be saturated with water to allow gravity flow to the lower layers. This excess - not retained by the soil and not absorbed by plants - can finally percolate downwards.
Only the force of gravity is then exerted on the flow of water. In spring and summer, the proportion of water that infiltrates in depth is non-existent, except during an exceptional rainfall event.
However, water does not flow vertically in a straight line and can take different paths through rock to reach the water table. The actual distance traveled depends on the effective porosity of the rock crossed, i.e. the existence of interconnected voids.
The speed of the water can be very slow if the water is flowing through poorly connected pores, fast if the water is flowing through cracks, fractures or go-karts. The infiltration rate can thus be of the order of one meter per year (chalk from the Paris Basin), one meter per month (Beauce limestones), one meter per day (alluvium) and several tens of meters. per day for very cracked karst-type systems (karstified limestone from the Cévennes border).
The transfer time to the web also depends on the thickness of the unsaturated zone crossed. The accompanying water tables, called alluvial water tables, are close to the surface of the soil in a humid valley. The maximum depth of a free water table in mainland France is around 80 meters on the chalk of the Picardy plateau.
When the water reaches the water table
After having cleared all these obstacles, the rainwater will have taken from a few hours to several months to reach the water table.
In spring and summer, a significant rainfall event may cause a momentary recharge episode, making it possible to maintain the levels, or even occasionally to observe an increase.
The trends will depend on local rainfall accumulations, evapotranspiration and water demands. However, these aquifers are also sensitive to prolonged drought. They drain quickly and can reach low levels in a few weeks in the absence of significant rainfall.
The flows between the surface and the slick are slower on inertial slicks, up to 2 to 3 months for chalk slicks and tertiary formations. These aquifers are not very sensitive to prolonged drought. In spring and summer, precipitation does not cause inertial water tables to recharge, except for very exceptional rainfall events.
Indeed, the water does not arrive at once (like a flushing of water) at the water table but in a way dispersed in time. This phenomenon then results in a slowing down of the emptying, several weeks after the rainfall event, which is difficult to detect on the levels of the water table.
Estimating the proportion of rain infiltrated at depth remains complex and difficult to quantify. It is therefore preferable to wait, from a few days to several weeks, to observe the impact of a rainfall event on the levels of the wells monitored.
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