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All right. To a new scientific study that shows trouble ahead for the world's streams and rivers. That is because people are draining underground aquifers that provide water to those streams. NPR's Dan Charles has more.
DAN CHARLES, BYLINE: Joshuah Perkin is a biologist at Texas A&M University. He studies fish. And he's spent a lot of time around water on the high plains in Kansas and Colorado. He's noticed an odd thing there - streams and rivers that disappear.
JOSHUAH PERKIN: We would go and visit these streams. And in many cases, it's like a dirt bike trail now. It's no longer functioning as a stream channel.
CHARLES: Those streams were partially fed by groundwater, water moving underground through rock and sand, draining into streambeds or bubbling up in natural springs. But people have been tapping those underground pools of water, drilling deep wells, mostly to irrigate farmland.
PERKIN: You know, growing crops in a region that's semi-arid and becoming increasingly arid. And the way that you continue to grow crops when there's no rain is to pull water from the ground.
CHARLES: Farmers have pumped so much water, the water table has fallen by more than a hundred feet in some places, so it can't flow into streams and rivers anymore, which means that fish and plants and birds around those streams also disappear. It's happened in other places, too - California, northern India, China.
Hydrologist Inge de Graaf at the University of Freiburg in Germany wanted to see the whole global picture, today and in the future.
INGE DE GRAAF: I think that's one of the goals that we have with this study - raising awareness to explain what is happening under our feet, so to say.
CHARLES: She and her colleagues created a computer simulation of groundwater and rivers all around the world. They threw in a heavy dose of climate change, an extreme scenario - lots of warming. In this computer model of the future, there's less rainfall. Farmers pump even more water to make up for that. So in places where farmers rely on groundwater, the water flow in streams and rivers in this model shrinks dramatically. In half of them, it falls below a kind of ecological limit.
DE GRAAF: Now the ecosystems cannot be sustained. The plants and the fish that live in the river or lakes - they will die.
CHARLES: Her paper appears this week in the journal Nature.
DE GRAAF: It's pretty alarming, but there's also things we can do about it to avoid it.
CHARLES: Slowing global warming, of course. Some places, like California, are starting to reduce the amount of groundwater that farmers are allowed to extract.
Dan Charles, NPR News.
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