I’ve seen the claim a lot, forests provide 75 percent of the world’s fresh water. It shows up on nonprofit infographics, tree planting campaigns, sustainability slideshows. I wanted to know if it actually holds up, or if it’s one of those numbers that gets repeated so often nobody checks it anymore.
The honest answer is that it depends on what exactly you’re measuring, and most of the places repeating it don’t say. The World Resources Institute puts a more specific number on it, about 31 percent of the world’s watershed area is currently covered by forest, and that share has been shrinking, with major watersheds losing roughly 6 percent of their tree cover between 2000 and 2014 alone. That’s a smaller, more technical number than 75 percent, and it’s measuring forest coverage of watershed land, not the share of usable freshwater that passes through forested ground before reaching a tap.
The U.S. Forest Service’s own numbers land somewhere in between, and they’re specific to the United States rather than the whole planet. Roughly 53 percent of the country’s surface water supply originates on forest land, even though forests cover only about 29 percent of the country’s surface area. National forests alone, managed under a single federal agency, supply water to more than 180 million people across over 68,000 communities. Cities that don’t think of themselves as forest adjacent at all, Los Angeles, Denver, Atlanta, Portland, still pull a real portion of their drinking water from forested land somewhere upstream.
So the 75 percent figure isn’t necessarily fabricated, but it’s doing a lot of unlabeled work. It might be measuring something closer to how much of the world’s accessible, usable freshwater passes through a forested watershed at some point in its journey, which is a different question than what fraction of watershed land is currently forested. Without a clear source attached, it’s hard to know which claim it’s actually making.
What isn’t in dispute is the mechanism, and that’s the part that actually matters more than the exact percentage. Adam Wei, a professor at the University of British Columbia Okanagan who studies forest hydrology, has spent years tracking what happens to water supplies when forest cover changes. His research team found that shifts in ground vegetation can affect regional water resources about as much as climate change itself, a comparison that surprised even people in his own field. As he put it, when we urbanize land and keep converting forests for other uses, our water regimes change. We end up with systems we didn’t design for, and entire watersheds get affected in ways that are hard to reverse.
Tree roots hold soil in place, which keeps rainfall from eroding straight into streams as sediment. Forest floor litter slows water down enough to let it soak into the ground and recharge aquifers instead of running off fast and dirty. None of that requires a specific percentage to be true. It just requires a forest to still be standing.
I don’t know the exact global percentage anymore, and neither, it turns out, does most of the internet. What I do know is that the actual mechanism behind the claim is real, well documented, and currently getting worse in a lot of places at once. That’s a less satisfying number to put on a slide. It’s probably the more honest one.
Sources. World Resources Institute; U.S. Forest Service; Wei, A., University of British Columbia Okanagan.