Continental waters have a crucial impact on terrestrial life and human needs, and play a major role in climate variability. Without taking into account the ice caps, fresh continental waters are stored in various reservoirs: the snow pack, underground reservoirs, the root zone (first few meters of the soil) and vegetation, and as surface waters (rivers, lakes, man-made reservoirs, wetlands and inundated areas). Water on Earth is continuously recycled through precipitation, evapotranspiration, runoff, and vertical and horizontal diffusion and transfer in soils.

An improved description of the global water cycle, especially, the poorly known continental domain, is of major importance for improved assessment and better management of water resources available for human consumption and other activities, as well as for climate prediction. Global monitoring of surface water requires products (i.e. lake and reservoir water levels and volumes, river levels and discharges, 3D floodplains water dynamic models) that may be derived from satellite datasets. Satellites now provide an essential component for the observation of the continental water from local to regional to global scales. Indeed, since the launch of Topex/Poseidon and ERS-1 in the early nineties until now, long term monitoring of water level on the lakes reservoirs, rivers and floodplains has been made possible thanks to the constant efforts and dedicated programs setup by several space agencies. It furthermore became evident and it has been demonstrated that the next generation of the surface water observing systems will depend upon in situ networks together with satellite constellation.

The next generation of higher resolution radar altimetry instruments exploiting new techniques such as along-track Delay-Doppler (Synthetic Aperture Radar) and interferometry (as in the CryoSat mission, enhanced for the SWOT mission and the CRISTAL mission) will permit a breakthrough in the monitoring of surface hydrological parameters. With more than two decades of exploitation of Radar Altimetry missions (ERS-1/2, Topex/Poseidon, Envisat, Jason-1/2/3, CryoSat, SARAL/AltiKa, Sentinel-3, IceSat2, Sentinel-6 Michael Freilich) the development and validation of river and lake level measurements has matured and will be further supported by a future generation of sensors (SWOT, CRISTAL, Sentinel Next Generation Topography Mission), for which the community is getting prepared, along with the systematic of use of optical and radar imagers data for volume variation, river width and river discharge, in conjunction with in situ observations and modelling, to improve our understanding of hydrological processes that affect river basins in response to climate variability and the management of water resources.

To meet the science, application and societal benefit objectives, the next challenges are to significantly improve modelling and forecasting skills through assimilation of observations, as well as the operational processes. Additionally, The new generation of instruments allows higher resolutions that will require new and improved processing algorithms, designing new products, training a new generation of scientists and eventually augmenting the user base for societal benefits.

A first workshop of this series was held in Toulouse, France, in 2003 (Click here for more information -- http://gos.legos.free.fr/HydroSpa2003.htm). The summary and recommendations, published in AGU's EOS, insisted on the organisation of a sequel meeting, which was held in Geneva in 2007 (Click here for the second workshop website -- http://earth.esa.int/workshops/hydrospace07/). A third workshop was held from 15 to 17 September 2015, ESA-ESRIN, Frascati (Rome), Italy (click here to view its website). It is again timely to gather the community around a workshop-style brainstorming event.