Historical droughts: what do the archives tell us?

In France, almost all departments experienced this summer 2022 water restrictions.

Some episodes of drought that occurred in previous decades and centuries, such as that of 1976, are still remembered. History thus remembers the drought of 1540 as the more severe in Europe known from documentary sources.

What repercussions have these events of the past had on societies and what is the interest of studying them with regard to our burning topicality?

Drought, a natural hazard and a social issue

Research on the drought generally focus on the atmospheric and terrestrial components of the water cycle. Drought is characterized first by a prolonged deficit of precipitation which can be accompanied by an increase in evapotranspiration in the event of very high temperatures: this is called “atmospheric” drought.

We then speak of “edaphic” drought, to describe a long period during which the water content of the soil remains very low (also called “agronomic” drought), and of “hydrological” and “hydrogeological” droughts in the event of low levels of soils. waterways (we speak of low water levels) and groundwater.

Drought phenomena can be greatly amplified by human factors: excessive abstraction, water diversion and intense soil drainage.

These events thus constitute complex phenomena and talking about drought is far from being unequivocal. This is a hazard whose perception is quite original compared to others (diffuse spatiality, absence of material damage or direct lethality, unlike floods in particular).

Observing and characterizing droughts

Methods for measuring and characterizing these different droughts have been developed at different times and are constantly evolving.

Although precipitation records have been taken since the Middle Ages, it was not until the middle of the XNUMXth century.^e century so that sets up a meteorological observation network in France. These instrumental measurements of precipitation and temperature make it possible to retrospectively characterize atmospheric drought using information such as the standardized precipitation index (SPI) or the standardized precipitation and temperature index (SPEI).

Continuous monitoring of water levels rivers dates from the end of the XNUMXth^e century and was primarily concerned with the levels reached during floods. The observation of low water flows is still today an issue that is often less of a priority than the monitoring of floods.

Similarly, the hydraulic markers marking the water levels reached during exceptional events essentially concern floods, with the exception of certain rivers in central Europe. We find there drought markers called "hunger stones", rocks usually submerged and on which the level of extreme low water levels has been engraved.

The characterization of hydrological drought is generally based on low water level indicators calculated from flows, but most of the hydrometric network was set up in the second half of the XNUMXth century.^e century. Reconstructions of historical series based on climatic archives can however be carried out in order to better understand and characterize past low water levels.

As for the floors, we also have to wait until the middle of the XNUMXth century.^e century to have a more exhaustive knowledge of their behavior during dry sequences.

In France, the CLIMSEC program traces its history back to 1958. Since the mid-twentieth^e century, a general drying up of soils in Western Europe is observed, which is in particular due to the increasing artificialization of surfaces. Coupled with the increase in temperatures measured on a global scale, this soil drying has the impact of increasing agronomic droughts, at least in Western Europe.

Why focus on past events?

The reconstruction of long hydro-climatological series is a concern of climatologists and hydrologists who seek reference events in the past for future extremes.

A reconstruction of low water levels since 1871 has for example been carried out by Laurie Caillouet on the scale of France and other works are available on regional scales. In the Moselle watershed, a retrospective study made it possible to highlight a series of low water levels between 1871 and 2018.

These reconstructions make it possible to reposition the current droughts over the long term and to have references to assess their severity.

It can thus be seen that extreme droughts as severe as the events of the summer of 2022 have already occurred in the past; but these episodes are now becoming more frequent and earlier.

This evolution (the processes are no longer constant over time) calls into question the idea of ​​stationarity of flows. However, the reference values ​​from which the rules for water management and the estimation of abstractable volumes are defined are based on frequency analysis methods which are based on the assumption of stationarity.

If the exceptional droughts experienced in recent years become the norm, then the reference values ​​will no longer be appropriate and will have to be reassessed taking into account these new trends.

“Remarkable” droughts

It is also by focusing on the social perceptions and representations of droughts that the historical perspective sheds valuable light.

Unlike other hazards (floods, earthquakes, gravitational movements), droughts have given rise to ambivalent reactions. Among meteorologists as among many farmers of the XIX^e century, long dry sequences were often interpreted or feared as a sign of climate change, which may seem like a curious premonition to our eyes.

But conversely, and no doubt because their most serious consequences are quite insidious (or badly appreciated by part of the population, especially in large cities), they have sometimes been considered with a certain curiosity, even with humor. .

In order to free themselves from the subjectivity of discourses or social reactions, climate historians have sought to categorize in order to compare drought events in history, by developing several indices.

Christian Pfister, a Swiss climate historian, has thus proposed an oft-used index that classifies certain periods (months, years) from -3 (very dry) to +3 (very wet), using data from the critical analysis of voluminous public or private archives.

But it remains possible to also take into consideration the social dimension of the event, as suggested by the work of Martin Boudou who proposes, for floods, developing a noteworthiness index, an approach adaptable to droughts. Its grid of parameters includes the intensity of the hazard, the severity of the damage and repercussions and the spatial extension of the event.

What the reference to historical droughts teaches us is that these events will now have to be taken much more seriously than hitherto (because of their increasing frequency and the growing inadequacy of our behaviors and lifestyles to this trend). It should also be emphasized that conflicts over water resources have been legion in the past and that their eventuality and their social motivations must be taken into account.

Blockages and tensions around water resources

On other scales of analysis, the archives show us that, from the XNUMX^e century, we reached situations of blockage for the use of water resources in agriculture as in industry. Before the rise of electricity or the steam engine, water was the first source of energy to run factories and a key factor in increasing agricultural yields.

On the occasion of certain dry sequences (at the beginning of the XNUMX^e century in the South or in 1857-58 in the east of France), and while the drought was less severe than today, and above all less associated with very high temperatures, conflicts broke out between industrialists who brandish the threat of unemployment and social disorder to run their factories using water and farmers forced by the authorities to restrict their watering.

Prepare to face

Public authorities, from the municipal to the national scale, were never indifferent to water shortages. In 1786, an intendant of the kingdom of France proposed in Touraine a spatial distribution of the parishes most affected after "a frightful drought which desolated our countryside and totally discouraged our ploughmen". The weight of excessive taxation is denounced in Anjou.

In the XVIII^e and XIX^e centuries, orders or decrees already authorize or limit water withdrawals, and the authorities organize rounds of the gendarmerie to check compliance with the measures, participate in water distributions, even launch surveys to find out the seriousness of the situation.

On June 22, 1893, information is taken by the prefecture of Indre-et-Loire to find out how the cattle are resisting. The municipal responses are very diverse (diverting rivers and flooding meadows, selling cattle, etc.) and sometimes alarming (cattle are dying and there is nothing to do).

In addition, structural measures are also envisaged very early on to deal with the risk of water shortage: they are sometimes very old, such as irrigation canals such as the bisses in Switzerland and even tanks.

1976, 2003, 2018 ...

Severe droughts constitute so many incentives or pretexts to build dams intended to support low water levels, which, for the environmental historian, reveals the strong economic interests linked to water.

In the Moselle basin, from the drought of 1921, surveys were carried out to find new resources for the supply of drinking water. But in this region, it is above all the feedback from 1976 that is at the origin of many works aimed at reducing the risk of water shortage (improvement of drinking water supply networks, creation deductions).

The year 2003 was also a pivotal event which highlighted the vulnerability of the whole territory to the risk of heat waves and drought. This results in the implementation of the heat wave plan and the drought plan which make it possible to better anticipate these events.

Studying the history of droughts and the political and economic reactions thus makes it possible to decipher the trajectories of the exploitation or management of water resources.

From now on, anticipated restrictive measures are decided, drought bulletins are published. More recently, and in view of more intense droughts, operational tools for forecasting low water levels (such as PREMHYCE projects et CIPRHOES) have been developed nationwide.

A comparison of the droughts of 1976 and 2018 showed that, in eastern France, 2018 was comparable to 1976 in terms of its intensity, but that, in 2018, the impacts were less, because the territory was better prepared and therefore less vulnerable.

This summer 2022, at the end of August, 79 French departments were in a “crisis situation”, which underlines the need for new measures to respond to the climate challenge, but also social and political. Because the local consequences of droughts raise questions: if the absence of rain is a triggering factor, certain social practices and political adjustments are not insignificant in explaining the lack of water.

History, whether climatic, hydrological, environmental or economic, should therefore not be overlooked. Water scarcity can increase inequities in access to the resource. Even a certain abundance is sometimes a source of inequality when the distribution is not equitable. It is a consequence of socio-political choices where climate variability is not always the most decisive.

Alexis Metzger, Geographer of the environment, climate and risks, INSA Center Val de Loire; Claire Delus, Lecturer in Geography, University of Lorraine; Judith Eeckman, Doctor in hydrology, University of Lausanne et Nicolas Jacob Rousseau, Lecturer in Geography, Lumière Lyon 2 University

This article is republished from The Conversation under Creative Commons license. Read theoriginal article.

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