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

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Dorigo, Wouter
Dietrich, Stephan
Aires, Filipe
Brocca, Luca
Carter, Sarah
Cretaux, Jean-François
Dunkerley, David
Enomoto, Hiroyuki
Forsberg, René
Güntner, Andreas
Hegglin, Michaela I.
Hollmann, Rainer
Hurst, Dale F.
Johannessen, Johnny A.
Kummerow, Christian
Lee, Tong
Luojus, Kari
Looser, Ulrich
Miralles, Diego
Pellet, Victor
Recknagel, Thomas
Vargas, Claudia Ruz
Schneider, Udo
Schoeneich, Philippe
Schröder, Marc
Tapper, Nigel
Vuglinsky, Valery
Wagner, Wolfgang
Yu, Lisan
Zappa, Luca
Zemp, Michael
Aich, Valentin
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Hydrologic cycle
Satellite observations
Surface fluxes
Surface observations
Water masses/storage
Water budget/balance
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 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, and resolution, to consistently 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.
Author Posting. © American Meteorological Society, 2021. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Bulletin of the American Meteorological Society 102(10), (2021): E1897–E1935,
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Dorigo, W., Dietrich, S., Aires, F., Brocca, L., Carter, S., Cretaux, J.-F., Dunkerley, D., Enomoto, H., Forsberg, R., Guntner, A., Hegglin, M., Hollmann, R., Hurst, D. F., Johannessen, J. A., Kummerow, C., Lee, T., Luojus, K., Looser, U., Miralles, D. G., Pellet, V., Recknagel, T., Vargas, C. R., Schneider, U., Schoeneich, P., Schröder, M., Tapper, N., Vuglinsky, V., Wagner, W., Yu, L., Zappa, L., Zemp, M., Aich, V. (2021). Closing the water cycle from observations across scales: where do we stand? Bulletin of the American Meteorological Society, 102(10), E1897–E1935.
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