The European Space Agency, in the context of the "Earth Observations Science for Society" Programme, and CNES are organising a sequel event to Hydrospace2015, called 4th Space for Hydrology Workshop - "Inland Water Storage and Runoff: Modeling, In Situ Data and Remote Sensing", Monday 7 - Friday 11 June 2021, hosted at ESA-ESRIN, Frascati, Italy. This time around the Hydrospace Workshop is organised in collaboration with the GEO Global Water Sustainability Initiative (GEOGloWS) - see the Objectives page - looking for opportunities to explore joint solutions with a broader view on key science questions for Research and Development, up to Operational use of remote sensing information. The aim of getting together during 4~4.5 days is to strengthen the collaboration between the four critical water communities: in situ, modelling and space observation scientists as well as “non-scientific” users such as water managers to address the needs of the application community.