It’s easy enough to measure rainfall, and nearly as easy to measure streamflow. Calculating the efficiency of water use through the metric of evapotranspiration (ET) – evaporation off the Earth’s surface and transpiration from the leaves of plants – is a far trickier proposal. In this episode of Eyes on Earth, we hear how scientists use satellites like Landsat to measure ET, and how those measurements help guide water management decisions in the U.S. and around the world.