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

Wouter Dorigo¹, Stephan Dietrich², Filipe Aires³, Luca Brocca⁴, Sarah Carter⁵, Jean-François Cretaux⁶, David Dunkerley⁷, Hiroyuki Enomoto⁸, René Forsberg⁹, Andreas Güntner^10,11, Michaela I. Hegglin¹², Rainer Hollmann¹³, Dale F. Hurst¹⁴, Johnny A. Johannessen¹⁵, Chris Kummerow¹⁶, Tong Lee¹⁷, Kari Luojus¹⁸, Ulrich Looser¹⁹, Diego G. Miralles²⁰, Victor Pellet²¹, Thomas Recknagel², Claudia Ruz Vargas²², Udo Schneider²³, Philippe Schoeneich²⁴, Marc Schröder¹³, Nigel Tapper²⁵, Valery Vuglinsky²⁶, Wolfgang Wagner¹, Lisan Yu²⁷, Luca Zappa¹, Michael Zemp²⁸, and Valentin Aich²⁹

1. TU Wien, GEO Department, Vienna, Austria 
2. International Centre for Water Resources and Global Change, German Federal Institute of Hydrology, Koblenz, Germany
3. LERMA, CNRS/Observatoire de Paris, Paris, France 
4. National Research Council, Research Institute for Geo-Hydrological Protection, Perugia, Italy 
5. Wageningen University and Research, Laboratory of Geo-Information Science and Remote Sensing, Wageningen, The Netherlands
6. Laboratoire d’Études en Géophysique et Océanographie Spatiales (LEGOS), Toulouse, France
7. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Australia
8. National Institute of Polar Research, Tokyo, Japan
9. National Space Institute, Technical University of Denmark
10. Helmholtz Centre Potsdam GFZ German Research Centre for Geosciences, Potsdam, Germany
11. University of Potsdam, Institute of Environmental Science and Geography, Potsdam, Germany
12. University of Reading, Department of Meteorology, Reading, United Kingdom
13. Satellite Climate Monitoring, Deutscher Wetterdienst, Offenbach, Germany
14. Cooperative Institute for Research in Environmental Sciences, University of Colorado, and NOAA Global Monitoring Division, Boulder, Colorado
15. Nansen Environmental and Remote Sensing Center and Geophysical Institute, University of Bergen, Norway
16. Colorado State University, Dept. Of Atmospheric Science, Fort Collins, Colorado
17. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California
18. Finnish Meteorological Institute, Helsinki, Finland
19. Global Runoff Data Centre, German Federal Institute of Hydrology, Koblenz, Germany
20. Hydro-Climate Extremes Lab (H-CEL), Ghent University, Ghent, Belgium
21. Institute of Industrial Science, The University of Tokyo, Tokyo, Japan
22. International Groundwater Resources Assessment Centre (IGRAC), Delft, The Netherlands
23. Global Precipitation Climatology Centre, Deutscher Wetterdienst, Offenbach a.M. Germany
24. University Grenoble Alpes, Institute for Urban Planning and Alpine Geography, Grenoble, France
25. School of Earth, Atmosphere and Environment, Monash University, Melbourne, Australia
26. Hydrological Institute, St. Petersburg, Russian Federation
27. Woods Hole Oceanographic Institution, Physical Oceanographic Department, Woods Hole, Massachusetts
28. University of Zurich, Zurich, Switzerland
29. Global Climate Observing System (GCOS), Geneva, Switzerland

Abstract

Life on Earth vitally depends on the availability of water. Human pressure on freshwater resources is increasing, as is human exposure to weather-related extremes (droughts, storms, floods) caused by climate change. Understanding these changes is pivotal for developing mitigation and adaptation strategies. The Global Climate Observing System (GCOS) defines a suite of Essential Climate Variables (ECVs), many related to the water cycle, required to systematically monitor the Earth's climate system. Since long-term observations of these ECVs are derived from different observation techniques, platforms, instruments, and retrieval algorithms, they often lack the accuracy, completeness, resolution, to consistently to characterize water cycle variability at multiple spatial and temporal scales.

Here, we review the capability of ground-based and remotely sensed observations of water cycle ECVs to consistently observe the hydrological cycle. We evaluate the relevant land, atmosphere, and ocean water storages and the fluxes between them, including anthropogenic water use. Particularly, we assess how well they close on multiple temporal and spatial scales. On this basis, we discuss gaps in observation systems and formulate guidelines for future water cycle observation strategies. We conclude that, while long-term water-cycle monitoring has greatly advanced in the past, many observational gaps still need to be overcome to close the water budget and enable a comprehensive and consistent assessment across scales. Trends in water cycle components can only be observed with great uncertainty, mainly due to insufficient length and homogeneity. An advanced closure of the water cycle requires improved model-data synthesis capabilities, particularly at regional to local scales.