Mass transports and redistributions within the System Earth are caused by a huge number of processes, such as circulations in the oceans and the atmosphere, water fluxes between terrestrial water storages, river discharge or sea level changes. In the past, it was rather difficult or even impossible to observe changes of the mass distribution. Today, however, simultaneously operating satellite gravity and altimetry missions allow for the detection of mass transports and redistributions. Consequently, satellite gravity missions such as GRACE are sensitive to the hydrological cycle. The hydrological cycle describes the continuous water transport on, above, and below the surface of the Earth. Freshwater represents only about 3% of all water on Earth. Around 68% of the freshwater is stored as snow and ice on Antarctica and Greenland; 30% are stored as ground water. Lakes, streams and rivers account for only 0.3% of the Earth's freshwater.