Dams and reservoirs: Their adverse effects in the event of long droughts

Heat waves and drought unprecedented in France and more broadly in continental Europe, with a succession of fires and the drying up of groundwater will, according to the projections of hydrologists, be more and more frequent, even the "new standard". These phenomena, which are accelerating, are also observed in different regions of the world.

Drought means a water deficit compared to a normal situation: it can be expressed by the level of precipitation, soil water or flow rates. We then speak of meteorological, agronomic or hydrological droughts, the three of which can occur at the same time, but often have their own dynamics.

These phenomena are directly impacted by climate change, and more frequent, more widespread and more intense droughts are expected in the future over a region. large part of the globe, and especially in France.

One million fish found dead in south-eastern Australia. The government attributes this massacre to the drought. But, according to Australian academics, this excess mortality is mainly due to the overexploitation of water. (Le Monde / YouTube, 2019).

Store water against drought

To fight against droughts, the water storage is often advocated as a solution. The idea is to make reserves with the water available in rainy periods (winter) in order to keep it for later uses (in summer).

This is reflected in particular in the desire to build dams or reservoirs, as indicated by the so-called territorial projects, aiming to store water and share it in the same area.

The world would number more than 16 million dams, totaling an estimated storage capacity of between 7 and 000 km³. That is 2 to 3 times the volumes of water contained in the rivers of the globe.

The quantities stored have increased sharply since the start of the XNUMXth century.e century, in order to meet an increasing demand for water for human activities. This made it possible to study the impact of these reservoirs, both on the resource and on demand, and to identify the consequences on the occurrence of droughts.

Evolution of the level of the Santa Juana reservoir in the Huasco basin in Chile. Crisis levels are shown in dotted lines. Right, a photo taken in 2014 during the multi-year drought that began in 2008.
Huasco Departmento Tecnico, 2014

By storing water during periods of abundance to promote its use during periods of low water, reservoirs reduce the impact of many floods and droughts.

Droughts amplified by human activity

This effectiveness is however limited to low intensity events. In fact, many studies show that the effectiveness of dams is very reduced for long droughts (such as, for example, in the Iberian Peninsula. as Austria or USA).

In northern China, over the past 30 years, human activities have amplified the severity and duration of droughts - some lasting up to more than two years. In Spain, the analysis of droughts between 1945 and 2005 showed that the most severe and longest dry episodes took place in the basins the most regulated by the presence of dams. This further leads to a intensification of droughts downstream from the basins.

It thus appears that the creation of large volumes of water storage for irrigation does not ensure a water supply. during long droughts, due to both the difficulty in filling the dams and the use of water that exceeds the resource.

The vicious circle of water addiction

The amplification of droughts by human activities is so marked that the notion of "anthropogenic" drought : it only involves dry episodes due to abstraction and management by reservoirs, regardless of climatic conditions.

Diagram of the occurrence of drought induced by climatic conditions alone (in yellow), by human activities (samples) alone (in dark), and by the combination of the two. In this case, drought occurs when the water level is below the threshold drawn in blue. The drought attribution is made by comparing the observed level (solid line) to a simulated level under natural conditions, without human activity, dotted.
“Drought in the Anthropocene”, _Nature Geoscience_, volume 9, pages 89–91 (2016)

This notion of human-induced drought is particularly well illustrated by the emblematic cases of large lakes, such as the aral sea and lake urmia in Iran, which are drying up because the water from the rivers feeding its lakes is diverted for human use.

This is not just a fatality, linked to a need for water, but of a vicious circle : a water deficit - that is to say a use of water greater than the resource - leads to socio-economic damage and generates pressure to create new water stocks: we then increase the reservoirs and the stored volumes.

But this gain in reserves is in fact offset by an increase in uses: for example, the increase in irrigated areas or population growth which increases the consumption of drinking water. These changes are aggravated by changing climatic factors. New water deficits then appear, and with them other socioeconomic damage.

Rather than developing strategies to reduce consumption, we induce increased dependence on water supply infrastructure: which increases vulnerability and economic damage in the event of water scarcity.

However, climate change almost certainly implies the multiplication of these deficiencies.

Reduce consumption to face droughts

Illustration of how water supply can worsen water scarcity: the figure represents, in blue, the supply-demand cycle, in brown, the influence of socio-economic pressures, and in pink, the growth of dependence and vulnerability linked to confidence in supply and adaptation deficit.

Studies are interested specifically to the impacts of water management on future droughts on a global scale. But they do not integrate this vicious circle mechanism, and therefore anticipate a constant number of reservoirs over time. Only the volumes for irrigation vary depending on the climate. In this context, which does not take into account the evolution of drinking water needs, the human arrangements will reduce by 10% the increase in agronomic droughts, that is to say, the water deficit in the soils, but will lead to a 50% increase in the intensity of droughts in rivers.

The multiplication of water reservoirs undermines their functionality, by limiting their capacity to filling, since more of them share the same limited resource. Even if these reservoirs are not collected for human activities, they experience evaporation losses, what reduces water resources.

These factors are particularly aggravated during long droughts, events unfortunately destined to become more frequent in the context of climate change. All actions to reduce water consumption will be beneficial in reducing our vulnerability to these events.

Florence Habets, CNRS research director in hydrometeorology, professor, École normale supérieure (ENS) - PSL

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

Image credit: Shutterstock.com / Piotr Mitelski

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