Where should we plant trees to improve freshwater availability?

The world’s vegetation cover has changed dramatically in the last two decades thanks to the twin forces of deforestation and afforestation, which is all affecting our freshwater systems.

According to new research published in Nature Geoscience, it’s led to an overall increase in water availability – but the story changes when you look region by region.

“The decision-making of afforestation programmes has never carefully considered the potential for unintended consequences on regional freshwater availability by such deliberate terrestrial changes,” write the international team of researchers in their paper.

“In particular, afforestation schemes do not account for the emerging observational evidence of declining water availability because of unsustainable water consumption by some ecological restoration projects.”

But while they require water to grow, forests also recycle that water back into the atmosphere through a process called evapotranspiration, making more water available again – both locally, and downwind, in areas where the predominant winds blow.

The researchers used satellite imagery to calculate “leaf area index”: a metric that allowed them to judge how much vegetation there was in a given area, from 2001-2018.

They then compared this map to local precipitation rates and atmospheric moisture data over the same period.

The results showed that globally, changes in vegetation have made more available freshwater at a rate of 0.26mm per year.

This has buffered the globe’s decrease in freshwater availability, making it about 15% less severe.

But afforestation specifically has a much more variable effect. The researchers have built a map showing how increased vegetation coverage influenced local water availability around the world.

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For 45% of the Earth’s surface, afforestation increases surface water both locally, and downwind of the afforestation projects.

This double-positive effect is clear in eastern China, Southeast Asia, the eastern Americas, Europe and eastern Africa.

“Planting trees in these key upwind areas may offer a co-benefit to carbon capture, with such afforestation supplying more water to mitigate local water resource scarcity,” write the authors.

But for 34% of the globe – including much of southern Australia, South Africa, the Middle East and western North America, afforestation decreases water availability locally – but increases it in downwind regions.

In 8% of the world, including New Guinea, the Tibetan Plateau, and western Canada, afforestation reduces both local and downwind water availability.

The researchers point out that, while afforestation is crucial to mitigating climate change through carbon sequestration, it’s worth examining where afforestation efforts can be best placed to conserve – or improve – water availability.

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