Against the backdrop of global change, both in terms of climate and demography, there is an increasing need for monitoring global water cycle. The publicly available global database is very limited in its spatial and temporal coverage worldwide. Moreover, the acquisition of in situ data and their delivery to the database are on the decline since the late 1970s be it for economical, political or other reasons. Given the insufficient monitoring from in situ gauge networks, and without any outlook of improvement, spaceborne approaches are currently being investigated. Satellite-based Earth observation with its global coverage and homogeneous accuracy has been demonstrated to be a potential alternative to in situ measurements. The Institute of Geodesy (GIS), within the Faculty of Aerospace Engineering and Geodesy at University of Stuttgart has a long-standing expertise, both theoretically and practically, in dynamic satellite geodesy. In recent years, GIS initiated and participated in studies and projects on application of spaceborne geodetic sensors for hydrological studies. HydroSat provides the results of these studies and projects, in which spaceborne geodetic sensors are used to estimate