The top 2 inches of topsoil on all of Earth’s landmasses contains an infinitesimal fraction of the planet’s water — less than one-thousandth of a percent. Yet because of its position at the interface between the land and the atmosphere, that tiny amount plays a crucial role in everything from agriculture to weather and climate, and even the spread of disease.
The behavior and dynamics of this reservoir of moisture have been very hard to quantify and analyze, however, because measurements have been slow and laborious to make.
That situation changed with the launch in 2015 of a NASA satellite called SMAP (Soil Moisture Active Passive), designed to provide globally comprehensive and frequent measurements of the moisture in that top layer of soil.
Read more at MIT News