The European Space Agency (ESA) DTE Hydrology project aims at fostering a fast step forward towards establishing a solid scientific and technical basis to realise a Digital Twin Earth focused on the water cycle, hydrology and its different applications. The project will capitalise on existing developments and incorporate the necessary elements to prototype a first instance of Digital Twin Earth for hydrological processes, as an integrated and interactive system providing the best possible reconstruction and simulations of the water cycle and the hydrological processes and its interactions with human activities at unprecedented resolutions and accuracies.