https://phys.org/news/2018-08-drought-co2-air.html

Using satellites to measure droughts

Plants are usually able to access water deep in the soil through their roots. However, conventional satellites only see what happens at the surface and cannot measure how much water is available underground. In the last few years, a new type of satellite mission has been used to measure extremely small changes in the Earth's gravity field. It was found that some small perturbations of the gravity field are caused by changes in water storage. When there is a major drought in a given region, there is less water mass and gravity is consequently slightly weaker over that region. Such variations are so small that they are imperceptible to humans. But by measuring them with satellites, scientists are able to estimate large-scale changes in water storage to an accuracy of about four centimetres everywhere on the planet.