For each kilogram of water on Earth, only 1 mg is stored as soil moisture. Yet this miniscule amount of water exerts significant control over various hydrological, ecological, and meteorological processes ranging from boundary layer dynamics to the global water cycle. The 1993 Mississippi River flood illustrated the power of 0.0001% of Earth's water to dictate the fate and evolution of regional weather and climate.
The space-time distribution of soil moisture is a key component in describing transfer and distribution of mass and energy between the land and the atmosphere. It is a fundamental variable in biosphere-atmosphere transfer, biogeochemistry, ecosystem process, and rainfall-runoff models. There is a growing consensus that a unified approach is necessary to monitor, characterize, and model soil moisture over a range of scales, but such an approach has yet to be defined.