Scientists stumble on a weird way to measure the moisture in soil.

“ARE these right?” Kristine Larson said. She was poring over raw Global Positioning System (GPS) data when she happened upon some curious signals in Los Angeles, California. The data spiked up along with a massive rainstorm in December 2004 that broke rainfall records and dumped more than five inches of rain in one day. Later, she searched for a clearer GPS signal somewhere else in the world, and found the same effect in Tashkent, Uzbekistan. “Did I just invent a rain gauge?” she wondered.

Her colleague, hydrologist Eric Small thought otherwise. “If you look at a rain gauge record, it’s either on or off,” he said. “Whatever signal Kristine was measuring was not over when the rain was over.” Small suspected that the GPS signals were interacting with something else. “It looked more like soil moisture to me,” he said. GPS had somehow recorded how drenched the soil was after the storm. It excited Small and Larson to think that ground-based GPS receivers—more commonly used for navigation—could possibly detect how much water is in the soil, a measure valuable for weather and flood forecasting that can be difficult to measure by any technique.

Originally published in Sensing Our Planet: NASA Earth Science Research Features. Read the full story here.