Carving Mountains and Ruining Picnics: The Power of Rain

There's one force in the universe that can both reduce a mountain to a meadow over a long period of time and mess up a Sunday picnic on a single afternoon, and until now we thought we knew all about it. We're talking raindrops here.

A single raindrop can hit a mound of dirt with nearly the force of a hammer, and that's one reason why soil erosion causes about $27 billion in economic losses in the United States each year. Researchers at Vanderbilt University in Nashville, Tenn., and Arizona State University in Tempe, Ariz., say raindrops are like "micro-missiles," striking the soil with such force that they can scatter particles in all directions.

Ever since the dust bowl days of the Great Depression, scientists have thought they had the processes there all worked out. And they were pretty much right on target, at least if the rain is falling on the plain. A paper published in the current issue of the Journal of Geophysical Research, however, reveals that on at least one critical point, they were wrong: When rain falls on a slope, the dynamics aren't what scientists had thought.

"We've known for a long time that if rain falls on a horizontal sedimentary soil surface, the rain imparts part of its momentum to the sand grains and they do this lovely dance all over the surface," says David Furbish of Vanderbilt, coauthor of the report with Mark Schmeeckle, an assistant professor of geography at ASU. The rain distributes the grains symmetrically around the impact crater.

Years ago, when the U.S. Department of Agriculture was trying to end the dust bowl, it didn't take a lot of sophisticated equipment to find out what happens when that raindrop falls on a sloping surface. The sand moved farther down the slope than up. The reason, scientists thought then, was obvious. The grains moving down the slope had to travel farther than those moving up the slope, or to either side of the impact crater, to hit the ground.

So for about half a century, nobody thought to challenge that obvious assumption. But one day when Furbish and Schmeeckle were colleagues at Florida State University, they started goofing off, to use a layman's term, with a high speed camera.

The experiment they had been working on left a little to be desired, so they turned their attention to rain.

"We grabbed this little container, and filled it full of sand," Furbish says. Using an eyedropper, he dropped tiny raindrops on the sand while Schmeeckle ran the camera, and the two later marveled at the patterns captured on film.

"We said, 'That's incredibly cool,'" Furbish adds.

Fast forward to the present, when both men have access to sophisticated laboratories. Schmeeckle provided a camera that can operate at 500 frames per second, thus freezing the dynamics of rain splash in a way that no one had seen before. The scientists conducted a range of experiments at Vanderbilt, confirming that when rain hits a flat surface, it scatters grains in all directions, as their predecessors had concluded decades ago.

But when they tilted the surface, they got a bit of a surprise. Very few of the grains were blasted up the slope. And the grains that traveled down the slope moved farther than those going up, but for a different reason.

Most of the grains were propelled down the slope, and to a greater distance, because most of the water also splashed downward. In other words, more grains moved downward and to a greater distance because they were driven by more water.

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