Scientists from Los Alamos National Laboratory and the University of California, Davis, are collaborating on an experiment this month to use laser technology to measure water evaporation from an agricultural field. The experiment will help UC Davis researchers improve their model of the interplay between the atmosphere and the land surface as it affects evaporation, the greatest source of water loss from agricultural fields. The research could ultimately help farmers reduce water use by irrigating their fields more efficiently. Currently, evaporation is measured by a series of ground stations located within the field or by weather balloons tethered above it. These instruments give spot measurements that must be averaged together to represent the evaporation from the entire field. The Los Alamos team will bring to a UC Davis experimental field a truck-mounted laser device -- called LIDAR for "light detection and ranging" -- that can measure the water content in the atmosphere above the entire field at once. "LIDAR measures water vapor in the air at many distances simultaneously, from the truck out to one kilometer," said David Holtkamp of Los Alamos. "With the data we can create a display of the water abundance from the ground up. It's a three-dimensional water map." Pulses of laser light sent out by the LIDAR truck are scattered by molecules in the atmosphere, in this case water vapor and nitrogen. By timing and measuring the laser light that reflects back to a detector on the truck, researchers can pinpoint the locations and amount of water vapor coming off the field. Earlier use of LIDAR showed that water vapor concentration varied dramatically from point to point above a field. The value of evaporation measured by ground stations or weather balloons, therefore, would depend primarily on the positions of the instruments. "To evaluate a field accurately with ground stations you would need a very large number of stations," said Rich Joseph, program manager for LIDAR research at Los Alamos. UC Davis Assistant Professor Marc Parlange expects that the LIDAR measurements will help with his work of modeling water loss through evaporation. "Climate and hydrological models are sensitive to the value we assign to evaporation," Parlange said. "After precipitation, evaporation is the largest term in the hydrologic cycle. It's also one of the hardest components of the cycle to describe. Evaporation is difficult to measure and model." LIDAR gets around that difficulty. "With our LIDAR system we can look at water vapor on a smaller scale than ever before and in a non-invasive fashion," Joseph said. "We can get a true picture of the fine-scale behavior of the interaction of water with the surface and the atmosphere."