| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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HORIZONS The Planet Also Rises Kathleen M. Wong The ground beneath your feet may not be nearly as solid as you once thought. Thanks to images taken from space, geologists have, for the first time, seen the surface of the Earth swelling after heavy rains like a soaked sponge. The new technique offers a relatively cheap and efficient way for people to make the most of an increasingly precious resource—fresh water. Wesley Danskin, a research hydrologist with the United States Geological Survey (usgs) in San Diego and colleague Zhong Lu, also with Raytheon, compared 13 high-resolution interferometric synthetic aperture radar, or InSAR, satellite scans of a watershed near San Bernardino, California, taken between 1992 and 1995. To obtain the data, the satellites directed a constant shower of radio waves toward Earth. The amount of time required for the waves to bounce back from the ground indicates the distance between the land and the satellite.“This is as close to magic as anything I’ve done,” says Danskin. The most dramatic changes in the land’s elevation appeared in the six months between December 1992 and August 1993, when exceptionally heavy rains deluged the nearby San Gabriel Mountains. Overwhelmed by torrents of water sheeting off the peaks, local rivers overflowed their banks. The satellite images revealed that the thirsty desert soils had absorbed enough rainwater in some areas to lift the surface of the ground by as much as three inches. “We were surprised at the amount of swelling in these areas. It’s one thing to know theoretically what happened, another to see it. It made believers out of us,” Danskin says.The basic idea that Earth expands and contracts because of changes in groundwater has been known for some time. When water soaks into the ground, it fills the tiny spaces between soil particles, causing them to float like the sand exposed by a receding wave. When groundwater is pumped out faster than it can be replaced, the soil particles shrink ever closer together. If too much groundwater is removed, the particles may never get sufficiently wet again to float, causing permanent land subsidence. This phenomenon is a major problem in California and much of the West, where places such as Modesto have sunk as much as 30 feet. Lu and Danskin noticed that a strange swelling in one area of the San Bernardino watershed persisted for months. Examining the images more closely, the scientists realized that they revealed the presence of hidden underground faults that were keeping the water from dissipating more quickly.“The fine sediments ground up in the fault zone act like a subsurface dam,” says Gerald Bawden of the USGS, who uses InSAR images to find buried earthquake faults. “In this one area, when it was really wet and rainy, this fault dam blocked all of the subsurface water flow, and the earth raised up in response, just like an inflating balloon.” In the parched West, the earth tends to absorb any river overflows almost immediately. Sinking down through the soil particles, the water may join an aquifer, a kind of hidden underground river that percolates through the sediments. Barriers such as impermeable faults, rock formations, and clays split up most watersheds into what are essentially adjoining drainage compartments. Mapping the locations of aquifer barriers and modeling the flow of groundwater is critical for watershed managers, who must offer a steady supply of water to customers while avoiding the ugly specter of subsidence. Traditionally, hydrogeologists have had to dig a series of expensive test wells and pump water across suspected aquifer boundaries to come up with the same information. InSAR won’t replace the use of good old-fashioned test wells, but it will save vast amounts of money by helping geologists identify promising areas to dig. Because experts can essentially deduce the plumbing of an entire drainage basin with a handful of InSAR images, the technique may offer the biggest boon to poor and remote communities who can’t afford to spend much on mapping their watersheds. By knowing which compartments of a basin recharge fastest after rain, and which are least likely to collapse when pumped, people from Southern California to the Sahara can better stretch their watershed’s supplies to last through a hot, dry summer—as well as year upon year of drought. Kathleen M. Wong is Senior Editor of California Wild. |