20 000 streams under the Earth

The management of water resources in the future is a major concern linked to climate change, and groundwater dynamics are an important element. Underground reservoirs store water over long periods of time and can supply rivers and wetlands or some human activities such as irrigation in time of need. These interactions, especially with the soil and the atmosphere, give them a particular place in the study of our climate and its evolution.

For the supply of water for human activities, such as drinking water production, crop irrigation or industry, underground resources are a gold mine. When it rains, some of it slowly seeps through the ground, and into the pores of the underlying rocks and fills these empty spaces. The storage capacity depends on the nature of the rock, which also affects how quickly the water reaches its destination. These aquifers store water over long periods of time and can supply rivers regularly, especially during periods of drought. This is very important for the biodiversity of aquatic environments but also for our use of water. “In our climates with few droughts it may seem obvious to have water in rivers, but it is not the norm and more arid environments, even in the Mediterranean, have many intermittent rivers,’ says Agnès Ducharne, researcher at METIS-IPSL.

These long residence times have an impact on the dynamics of water exchanges, in vapour form between continental surfaces and the atmosphere and in liquid form with rivers and wetlands. The interactions of water table with the climate and its evolution, but also with human activities, are multiple and interconnected. Mainly, they are a subject of study at the heart of the water cycle and the management of this resource in the future, one of the major concerns linked to climate change. “This long memory system also acts as a buffer against extreme events,” she adds. By intercepting and storing part of the water during extreme rainfall events, these underground reserves can play an important role in flood dynamics. At the same time, if these reservoirs are saturated, surface water will not be able to infiltrate and will result in flooding by adding to runoff.

More comprehensive models

Many local observations provide information on such things as water levels or base flow in rivers. But impressive more indirect methods have also been developed to estimate the distribution of water underground, notably by spatial gravimetry. This data feed models to better understand the processes involved and the links between groundwater and climate variations. “I really enjoy carrying out numerical experiments in this sense,” says Agnès Ducharne. “We can run simulations to see what happens with and without groundwater to understand its role on climate and its importance at a local level,” she adds.

“We have recently been able to show with these experiments that only groundwater that is sufficiently close to the surface can have an impact on the local climate, even if it is not very strong,” she says. Through experiments she can also study the effect of irrigation, trying out different sources of supply and how this may come in conflict with climate change. Modelling is an ideal tool to study the impact continental hydrology can have on the climate system and its evolution. However, current models do not generally take into account all the variables and processes involved in continental hydrology.

In order to associate the interactions between water tables, the soil and the atmosphere, Agnès Ducharne works with the ORCHIDEE continental surface model, which she has provided with a fairly simplistic representation of groundwater. This is currently the case for all climate models, where the description of groundwater or irrigation, for instance, is still rather coarse if not absent. On the contrary some models, known as hydrogeological models, are very precise in their description of groundwater and its interactions with surface water, but do not describe interactions with climate. The researcher is thinking in particular of the AquiFR platform coordinated by Florence Habets, also an IPSL researcher at the ENS. “It is a very interesting tool that combines different and sometimes very refined models and harmonises them to map and assess temporal changes, particularly in water table levels, in areas of France.” However, this type of model does not allow the study of the impact of groundwater on the climate. “Ideally, these two approaches should one day be combined to obtain more realistic models on a global scale,” stresses Agnès Ducharne. This would provide a more detailed understanding of groundwater dynamics and interactions with climate leading to more detailed projections of climate change.

Looking ahead

That said, current climate change scenarios already provide information on several aspects. “In general, the impacts of climate change on groundwater resources are in line with surface water in a given region,” sates Agnès Ducharne. Climate change tends to increase evaporative demand by increasing the radiation received at the surface and warming the atmosphere. “Over the oceans it increases evaporation. But for evaporation to increase over the continents precipitation levels must follow so that water is available. And climate change is intensifying the existing situation: precipitation will increase in regions where it rains a lot, but decrease in dry regions,” she points out. This will intensify the contrasts between areas with high or even too high water availability and others that are or will become too dry. This is obviously not ideal for the various uses of water whether it is for drinking, industrial use, energy production or irrigation.

“Today’s climate projections, and therefore those that will be included in the IPCC report this summer, rarely take account of groundwater or irrigation,” mentions the researcher. When these components are taken into account it is done in a very simplistic way with a relatively uniform description across the continents, because it is extremely difficult to characterise them properly. “Moreover, we do not yet have any climate projections that include both irrigation and groundwater,” notes Agnès Ducharne. This combined depiction is the subject of the international BLUEGEM project that she is coordinating to offer new perspectives on the uncertainties of climate projections. “Through two models, the IPSL model and the American CESM2 model, we want to explore the modification in climate change trajectories that can be obtained by taking into account the interactions between climate, groundwater and irrigation,” she explains. But also the possible social repercussions that it entails, which are essential to identify sustainable trajectories on issues related to water resources, food security and biodiversity.

Agnes Ducharne