Closing the water cycle from observations across scales: Where do we stand?

Life on Earth is intimately connected to the availability of water in space and time. With a growing world population and living standards, human pressure on freshwater resources is continuously increasing, as is the exposure of humans to weather and climate-related extremes like droughts, storms, and floods. This is further exacerbated by climate change, e.g., by shifts in precipitation patterns or the intensification of extreme events. Hence human access and vulnerability to water resources is increasingly being challenged.The Global Climate Observing System (GCOS) defines a suite of Essential Climate Variables (ECVs), many of them related to the water cycle, that are required to systematically observe the Earth's changing climate. However, since long-term observations of these ECVs are typically derived from different observation techniques, platforms, instruments, and independent retrieval algorithms, they often lack the accuracy, completeness, resolution, and consistency required to characterize climate variability at multiple spatial and temporal scales. While climate change often manifests itself in subtle, yet impactful modifications of the water cycle, detecting them remains challenging.Here, we assess the capability of available ground and Earth observations of water cycle ECVs to consistently monitor the variability of the hydrological cycle at various spatial and temporal scales. We assess the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Based on the assessment, we discuss gaps in existing observation systems and formulate guidelines for future water cycle observation strategies.