A paradox of our times is that in the space of just a few months, populations have found themselves the victims of both serious flooding and water restrictions. In the Pas-de-Calais département, the historic floods of November 2023 and January 2024 followed the widespread drought of the summer of 2022, which led to the imposition of water cuts and restrictions on groundwater abstraction.
The main cause is climate change, which is intensifying the water cycle and increasing the frequency and severity of hazards. To better anticipate and understand these extreme events, which are bound to increase in the years to come, groundwater studies are crucial.
This is because groundwater feeds rivers and can cause them to overflow. It also supplies 96% of drinking water catchments nationwide, accounting for two-thirds of the volumes used.
What is hydrogeological modelling?
Piezometers are used to monitor changes in groundwater levels. These are boreholes fitted with a probe to measure the water level (or piezometric level) in aquifers. BRGM monitors the national piezometric network. As such, it manages 1,710 piezometers, which have been monitored for an average of 30 years, although some piezometric measurements are much older, with wells that have sometimes been measured manually for more than a hundred years.
It uses models to forecast changes in river flows and groundwater levels over the coming months, in order to anticipate risks of drought or flooding due to rising groundwater.
A model is a simplified representation of reality that aims to describe, explain or predict complex phenomena by emphasising certain elements while excluding others. It enables observations to be put into an objective perspective, hypotheses to be tested and predictions about the behaviour of systems to be made, while making it easier to understand them. However, the effectiveness of a model depends on a delicate balance between simplification and the extent to which it corresponds to a real situation.
For groundwater, a numerical model can be used to take account of the various factors that play a major role in the behaviour of an aquifer (nature of the geological substratum, surface relief, land use, effect of human activity, meteorology, etc.).
Are there any forecasts for the state of aquifers?
In France, the MétéEAU Nappes and Aqui-FR modelling platforms are used to forecast seasonal variations in groundwater levels. These two platforms have been operational for several years and are still being improved and extended.
However, despite the uncertainties inherent in any modelling exercise, these tools are proving useful for anticipating periods of crisis and facilitating decision-making. The results of simulations and forecasts also provide a basis for discussion involving scientists, managers and the general public.
