Water security is no longer the problem of a few regions; it concerns all global economies, rich and poor. This stresses the need for a holistic (integrated) approach to water resource management and decision making, including defence and mitigation of climatic extremes (floods and droughts) and associated impacts of food and water security, energy, and large-scale (re)forestation. However, both the availability and quality of water resources, as well as how these change in time, are difficult to estimate in many regions of the world. This is particularly due to the fact that many low-income countries lack basic monitoring systems. On the other hand, a multitude of Earth observations (EOs) are available, providing global-scale monitoring of water cycle parameters, and research based on EO and in situ data is ongoing.

Current research focusses on the consistent integration of water resource datasets, including those derived from Earth observations, in situ measurements, and hydrological models, to facilitate water resource assessments worldwide. In addition, effort is devoted in several research investigations to downscale these datasets and demonstrate their uses in case studies at regional, national, and local levels around the globe. Furthermore, large-scale models are being used more and more in local studies.

We invite submissions of original papers and reviews to this issue that relate to the list of subjects below. The proposed special issue is open to all papers that fit within the scope of the special issue. We expect about 50% of the contributions to come from the eartH2Observe project and the other 50% from outside the project.