LE FIGARO. – How have we altered the water cycle?

CHARLÈNE DESCOLLONGES – We learned that the water cycle was closed, natural and independent of human activities. This is false, humans have always sought to convey it, channel it. These alterations relate to three components forming our water footprint: gray water – to dilute released pollutants; blue water – from rivers, aquifers, etc. -, which has been disrupted by excess withdrawals and the impact of hydraulic developments on the natural hydrology of watercourses, and green water – contained in soils and evapotranspiration by plants to grow, appropriated by deforestation, intensive agriculture, etc. Green water and blue water constitute two of the seven planetary limits already crossed. When rain falls, it evaporates, infiltrates or runs off the surface. With these disturbances (intensive agriculture, urbanization, artificialization of soils, destruction of wetlands), we have accelerated the water cycle: soils that have become like concrete are no longer capable of absorbing and therefore retaining water, which flows very quickly to rivers and the sea.

Is climate change making these disruptions worse?

We disrupted the water cycle long before disrupting the climate, but climate change will accentuate these effects. The more greenhouse gases we emit, the more water we evaporate. These vapor flows go into the atmosphere and then precipitate: this acceleration generates long periods without precipitation and periods of more intense and frequent rain. While all our uses have been adapted to the available resource, this availability will change over time and space. However, rain of 50 mm per hour will not have time to infiltrate – especially in dead and impermeable soils – like rain of 5 to 6 mm per hour.

What is regenerative hydrology?

Good news, there are solutions. The first: stop the waterproofing of the floors so that water infiltrates again. This is the purpose of the zero net artificialization law, which must be applied by local elected officials. Certainly it questions the suburban model, dear to the French, but we need ambitious policies to densify cities. On already artificialized soils, we can de-waterproof it by creating “sponge cities”. The difficulty is access to land. Today this type of project is mainly carried out in the public domain. We must also succeed in mobilizing private actors, promoters or groups of owners.

Regenerative hydrology is the other part of the solution, on agricultural and forestry lands. The goal of this emerging discipline is to slow down the water cycle at the watershed scale. This requires land planning that slows runoff using nature-based solutions, such as swales, small vegetated trenches perpendicular to the slope to break it up and prevent water from flowing down. Beyond the relief, it relies on vegetation: planting hedges that retain water, developing agroforestry… This is combined with regenerative agriculture, which restores degraded soils more capable of infiltrating and storing water. water in the soil, by giving them life again (by covering the soil, increasing the level of organic matter, recreating mycorrhizal systems, etc.). With these approaches, we can improve resilience to droughts, soil erosion and floods.

What is the link between forests and the water cycle?

Forests will regulate temperature, better capture water and infiltrate it into groundwater; they also have the power to condense and recycle rain. This is the theory of the biotic pump, which hydrologists are beginning to integrate: on a global scale, two thirds of continental rain comes from the evapotranspiration of plants, the rest from the oceans. The more diversified forests we have, the more they will capture these flows of water vapor and retain them so that the water can be recycled on the continents.

Do we already have any feedback?

Originally, regenerative hydrology was a simple reading of the landscape, developed by an Australian farmer. Then these principles inspired the inventors of permaculture, and were taken up in Canada, India and Kenya. In Slovakia, it has provided greater resilience to drought. We want to launch pilot projects in France around the Rhône.

* Author of the book “Water. Fake or not”, Tana, 2023.

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