Billions of gallons of freshwater are used every day to irrigate crops, but a lot of it gets wasted on already ripe or dying plants. Now, researchers have used images captured by an unmanned aerial vehicle (UAV)—i.e., a drone—to map barley fields and determine which rows of plants are most in need of water. The team mounted cameras sensitive to visual and infrared light—to collect both optical and thermal information—on a battery-powered UAV and flew the drone (pictured) 90 meters above fields in Denmark. With the airborne imagery, obtained in spring and summer, the researchers measured the greenness and temperature of the barley plants (temperature reflects the water content of nearby air and soil) and calculated the water stress level of each 25-by-25-centimeter patch of field, as they report in Biogeosciences last month. The researchers validated their results using measurements of soil water content obtained directly from the fields, and they showed that their airborne observations could reliably differentiate between ripe crops—which need less water—and unripe crops. Water stress maps like these can pinpoint the plants most in need of irrigation, potentially allowing farmers to minimize their water use and reduce pollutant runoff.