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The Seine basin: the effects of climate change are already here

From the Côte-d’Or, where it springs, to Le Havre where it flows into the Channel, the Seine crosses no fewer than 400 municipalities. Its basin covers 12% of the surface area of France and accommodates a mostly urban population of 18 million, in other words a quarter of the French population. 40% of France’s industry and 25% of its agriculture are concentrated in this basin, which generates more than half the national river traffic. All these figures underline the specificity of this basin: the heavy pressure placed on its water resources and on the life of the habitats within it. And then there are the impacts of climate change that are bound to make even deeper marks.

Let’s beam forward to 2100:

The temperature of the air has increased by 2 to 3°C, evapotranspiration is higher (+23%) while rainfall has decreased by 12%, without however leading to a lower risk of flooding. The sea level has risen by a metre. Naturally, all these phenomena have an impact on the Seine: its water is warmer (up by an average of 2°C) but of lower quality and with a higher concentration of pollutants; the water table has lowered; the discharge of the Seine, which has always been sluggish (the low water flow is currently 91 m3/s) has fallen by about 30% and the low flow periods are more severe.

30% of the water in the Seine basin will disappear from now to 2100

This is not the scenario of a disaster movie but the result of forward studies written by scientists. Furthermore, all the actors of the Seine that IFGR has met recently confirm it: the effects of climate change on this highly sensitive river are already apparent.

The SIAAP is already treating 2,400,000 m3 of wastewater and rainwater every day for slightly more than 9 million inhabitants. Management and regulation are increasingly pushed to the limit with more frequent extreme climatic episodes. To control flood risks and accidental pollution in the Seine or the Marne due to overflows of rainwater, the SIAAP has equipped itself since 1997 with a water retention capacity of 900,000 m3 comprising 4 reservoir tunnels and 8 storage basins. Ever increasing urbanisation makes the natural absorption of rainwater by soil more difficult, thereby augmenting the volumes of water to be treated.

Seine Grands Lacs, which operates the four lake-reservoirs of the Seine basin, plays a vital role in ensuring water resources for different uses (navigation, drinking water, agriculture, industry, etc.). Low water replenishment had to be adjusted in 2016, a year that saw a severe drought during winter followed by exceptional flooding in spring. In mid-November, only 91 million m3 remained in the lakes, a volume equal to 11% of the normal storage capacity.

How is it possible to anticipate and adapt to these changes?

The scientific community, elected representatives, farmers, industrial companies and other users must work together. 283 organisations have already signed the commitment to adapt the basin, a strategy set up in December 2016 by the Seine-Normandie basin committee and the coordinating Prefect to preserve water resources and ensure a healthy living environment and resilient ecosystems.

The PIREN-Seine, an interdisciplinary research group of which Ghislain de Marsily, an IFGR member, was one of the founders, is also focusing on the issue. Since 2015, he has been drawing up an inventory of the basin’s water resources and working to understand the mechanisms that regulate them in order to propose management practices adapted to the agricultural, environmental and urban challenges facing the basin.

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