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

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 Güntner10,11, 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


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.