EGU General Assembly 2023
© Author(s) 2023. This work is distributed under
the Creative Commons Attribution 4.0 License.

Functional relationships reveal differences in the water cycle representation of global water models

Sebastian Gnann1, Robert Reinecke1, Lina Stein1, Yoshihide Wada2,3, Wim Thiery4, Hannes Müller Schmid5,6, Yusuke Satoh7, Yadu Pokhrel8, Sebastian Ostberg9, Aristeidis Koutroulis10, Naota Hanasaki11, Manolis Grillakis10, Simon N. Gosling12, Peter Burek3, Marc F. P. Bierkens13,14, and Thorsten Wagener1
Sebastian Gnann et al.
  • 1Institute of Environmental Science and Geography, University of Potsdam, Potsdam, Germany
  • 2Climate and Livability, Biological and Environmental Science and Engineering Division, King Abdullah University of Science and Technology, Thuwal, Saudi Arabia
  • 3International Institute for Applied Systems Analysis, Laxenburg, Austria
  • 4Department of Hydrology and Hydraulic Engineering, Vrije Universiteit Brussel, Brussels, Belgium
  • 5Institute of Physical Geography, Goethe University Frankfurt, Frankfurt am Main, Germany
  • 6Senckenberg Leibniz Biodiversity and Climate Research Centre (SBiK-F), Frankfurt am Main, Germany
  • 7Moon Soul Graduate School of Future Strategy, Korea Advanced Institute of Science and Technology, Korea
  • 8Department of Civil and Environmental Engineering, Michigan State University, East Lansing, MI, USA
  • 9Potsdam Institute for Climate Impact Research (PIK), Member of the Leibniz Association, Potsdam, Germany
  • 10School of Chemical and Environmental Engineering, Technical University of Crete, Greece
  • 11National Institute for Environmental Studies, Tsukuba, Japan
  • 12School of Geography, University of Nottingham, Nottingham, United Kingdom
  • 13Department of Physical Geography, Utrecht University, The Netherlands
  • 14Unit Soil and Groundwater Systems, Deltares, Utrecht, The Netherlands

Global water models are widely used for policy-making and in scientific studies, but substantial inter-model differences highlight the need for additional evaluation. Here we evaluate global water models by assessing so-called functional relationships between system forcing and response variables. The more widely used comparisons between observed and simulated fluxes provide insight into model behavior for the representative area of an observation, and can therefore potentially improve the model for that area. Functional relationships, by contrast, aim to capture how system forcing and response variables co-vary across large scales, and thus offer the potential for model improvement over large areas. Using 30-year annual averages from 8 global water models, we quantify such functional relationships by calculating correlations between key forcing variables (precipitation, net radiation) and water fluxes (actual evapotranspiration, groundwater recharge, total runoff). We find strong disagreement for groundwater recharge, some disagreement for total runoff, and the best agreement for evapotranspiration. Observation- and theory-derived functional relationships show varying agreements with models, indicating where model representations and our process understanding are particularly uncertain. Overall, our results suggest that model improvement is most important for the representation of energy balance processes, recharge processes, and generally for model behavior in dry and cold regions. We argue that advancing our ability to simulate global hydrology requires a better perceptual understanding of the global water cycle. To evaluate if our models match that understanding, we should explore alternative evaluation strategies, such as the use of functional relationships.

How to cite: Gnann, S., Reinecke, R., Stein, L., Wada, Y., Thiery, W., Müller Schmid, H., Satoh, Y., Pokhrel, Y., Ostberg, S., Koutroulis, A., Hanasaki, N., Grillakis, M., Gosling, S. N., Burek, P., Bierkens, M. F. P., and Wagener, T.: Functional relationships reveal differences in the water cycle representation of global water models, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-2001,, 2023.