Terrestrial waters—including snowpack, glaciers, water in aquifers and other geological formations, water in the plant root zone, rivers, lakes, man-made reservoirs, wetlands, and inundated areas—represent less than a mere 1% of the total amount of water on Earth. However, they have a crucial impact on terrestrial life and human needs and play a major role in climate variability.

Land waters are continuously exchanged with the atmosphere and oceans in vertical and horizontal mass fluxes through evaporation, transpiration, and surface and subsurface runoff. Although it is now recognized that improved description of the terrestrial branch of the global water cycle is of major importance for climate research and for inventory and management of water resources, the global distribution and spatial-temporal variations of terrestrial waters are still poorly known because routine in situ observations are not available globally. So far, global estimates of spatial-temporal change of land water stored in soils and in the snowpack essentially rely on hydrological models, either coupled with atmosphere/ocean global circulation models and/or forced by observations.