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 yet poorly known continental branch, is of significant importance for inventory and better management of water resources available for human consumption and activities (agriculture, urbanization, hydroelectric energy resources, tourism, domestic use), as well as for biodiversity preservation and predictions to address disaster risk reduction. Observations, both satellite and in situ, are vital for understanding and creating solutions to the issues related to hydrology. The use and integration of these observations are critical for getting the best accuracy and best utility of both worlds.

Satellites are an essential component of the observation, providing an understanding of the relations among the regional, continental, and global scales. The monitoring of water level on lakes, reservoirs, rivers, and floodplains has been made possible thanks to the constant efforts and dedicated programs set up by several space agencies. The current and future generations of higher resolution radar-altimetry instruments, such as along-track Delay-Doppler (SAR) and interferometry (SWOT), are transforming the monitoring of surface hydrological parameters. With a new generation of instruments, images of higher resolution are obtained, requiring the development of new algorithms and the training of a new generation of scientists. It is also imperative to work towards satellite data “analysis-ready,” and develop the technical skills needed to interpret the data and translate it into meaningful information that conforms to essential requirements of accuracy and utility to support policies and programs.

The Hydrospace conferences  (2003 in Toulouse, 2007 in Geneva, and 2015 in Frascati) have traditionally focused on continental surface water monitoring using satellite techniques (altimetry, radar, and optical sensors at different resolutions) and hydrologic models. Since the last Hydrospace conference in 2015, some products such as water level in lakes, reservoirs and rivers, flood extent and volume, river discharge, floodplain deltas, and estuaries 3D water dynamic are now delivered in an operational mode; others still need further developments, and others are just emerging as new products.

The Group on Earth Observations Global Water Sustainability (GEOGloWS) Initiative (www.geoglows.org) works to provide relevant, actionable water information to promote the use of earth observations in the decision-making process. Through partnerships, GEOGloWS leverages organizations' capabilities for projects that complement national efforts and provide information where little or none exists to achieve its mission. One of these collaborations includes organizations such as ECMWF, NASA, NOAA, Brigham Young University, Esri, Aquaveo, the World Bank, and many National water organizations that have fuelled the streamflow forecasting services' technological development. These activities facilitate scientists' collaboration across fields to promote resource and project sharing while responding to user requirements in operational environments.

Considering the complementarity of GEOGloWS and the Hydrospace activities, this joint conference represents an opportunity to explore joint solutions with a broader view, and to address key issues including:

• What are the new key science questions? What are the new challenges and how should we address them?
• What are the new algorithms and the new advancements allowing the use of satellite data with the most advanced models in particular for ungauged basin? How can we benefit from the new solutions offered by online super-computers?
• What aspects of surface water observations and modelling are sufficiently mature for use in operational services?
• Do we need new types of instruments? How can we extract new knowledge from the new missions ahead (Sentinel-6A, SWOT, Sentinel-3C/-3D, WISA, others…)? How do we take advantage of all available data and give access for hydrologists to develop useful products?
• Could we improve the spatial and temporal coverage by altering scanning strategies (i.e., wider swaths of data) or by employing some sort of satellite constellation concept rather than live with very long repeat coverage?
• How do we fill the gap between Research and Development and Operational Use of remote sensing information in hydrological applications, forecasting operational system, and water resources management?
• How do we strengthen the collaboration between the four critical water communities: in-situ, modelling, space observation scientists and “non-scientist” users? Who the “non-scientist” users are and what they need is still an issue only partially addressed. Can we collectively do better? What are the new capabilities of space-based data for the application community?

The expected outcome of the workshop is to define an action plan for the future and converge on recommendations from the Scientific Community. A round table discussion is planned to cover the aforementioned seed-questions.