Artificial watershed provides "real" data.

• Position: Weather Models

Rain in southern Arizona is scarce and precious to begin with, but the afternoon shower that soaked the soil 25 miles north of Tucson on Nov. 29 was unusual in several ways. Spouting from a network of pipes, thousands of gallons of water drizzled down onto the world's only man-made experimental watershed, recently completed at the University of Arizona's Biosphere 2. Six-hundred tons of ground-up volcanic rock blanket a giant steel tub resting at an incline to form an artificial hillslope. Three identical such hillslopes, each measuring 100 feet long and 40 feet wide, were constructed side by side to form the Landscape Evolution Observatory, or LEO, with the first now fully functional.

Each slope is studded with a network of more than 1,800 sensors and samplers tracking the flow of water through the soil and drawing samples for analysis. "It's the first time anyone has built an instrument like that," marvels Peter Troch, director of Biosphere 2 and professor in the Department of Hydrology and Water Resources. "LEO provides the scientific community with a tool to learn about the landscape in ways we haven't been able to before. It will help us to really understand Earth's surface processes.

"The interactions of water and organisms living in and on the ground are what sustains life on Earth," Troch adds, explaining that minerals in the soil release nutrients, which then are transported in water and changed by microbes until plants extract them and turn them into biomass.

"We don't know much about those processes. Geochemists...