Submitted as: review and perspective paper 20 Apr 2021

Submitted as: review and perspective paper | 20 Apr 2021

Review status: this preprint is currently under review for the journal GMD.

GMD Perspective: the quest to improve the evaluation of groundwater representation in continental to global scale models

Tom Gleeson1,2, Thorsten Wagener3, Petra Döll4, Samuel C. Zipper1,5, Charles West3, Yoshihide Wada6, Richard Taylor7, Bridget Scanlon8, Rafael Rosolem3, Shams Rahman3, Nurudeen Oshinlaja9, Reed Maxwell10, Min-Hui Lo11, Hyungjun Kim12, Mary Hill13, Andreas Hartmann14,3, Graham Fogg15, James S. Famiglietti16, Agnès Ducharne17, Inge de Graaf18,19, Mark Cuthbert9,20, Laura Condon21, Etienne Bresciani22, and Marc F. P. Bierkens23,24 Tom Gleeson et al.
  • 1Department of Civil Engineering, University of Victoria, Canada
  • 2School of Earth and Ocean Sciences, University of Victoria
  • 3Department of Civil Engineering, University of Bristol, UK & Cabot Institute, University of Bristol, UK
  • 4Institut für Physische Geographie, Goethe-Universität Frankfurt am Main and Senckenberg Leibniz Biodiversity and Climate Research Centre Frankfurt (SBiK-F), Frankfurt am Main, Germany
  • 5Kansas Geological Survey, University of Kansas
  • 6International Institute for Applied Systems Analysis, Laxenburg, Austria
  • 7Department of Geography, University College London, UK
  • 8Bureau of Economic Geology, The University of Texas at Austin, USA
  • 9School of Earth and Environmental Sciences & Water Research Institute, Cardiff University, UK
  • 10Department of Geology and Geological Engineering, Colorado School of Mines, USA
  • 11Department of Atmospheric Sciences, National Taiwan University, Taiwan
  • 12Institute of Industrial Science, The University of Tokyo
  • 13Department of Geology, University of Kansas, USA
  • 14Chair of Hydrological Modeling and Water Resources, University of Freiburg, Germany
  • 15Department of Land, Air and Water Resources and Earth and Planetary Sciences, University of California, Davis, USA
  • 16School of Environment and Sustainability and Global Institute for Water Security, University of Saskatchewan, Saskatoon, Canada
  • 17Sorbonne Université, CNRS, EPHE, IPSL, UMR 7619 METIS, Paris, France
  • 18Chair or Environmental Hydrological Systems, University of Freiburg, Germany
  • 19Water Systems and Global Change Group, Wageningen University, Wageningen, Netherlands
  • 20School of Civil and Environmental Engineering, The University of New South Wales, Sydney, Australia
  • 21Department of Hydrology & Atmospheric Sciences, University of Arizona, Tucson, Arizona, USA
  • 22Center for Advanced Studies in Arid Zones (CEAZA), La Serena, Chile
  • 23Physical Geography, Utrecht University, Utrecht, Netherlands
  • 24Deltares, Utrecht, Netherlands

Abstract. Continental- to global-scale hydrologic and land surface models increasingly include representations of the groundwater system. Such large-scale models are essential for examining, communicating, and understanding the dynamic interactions between the Earth System above and below the land surface as well as the opportunities and limits of groundwater resources. We argue that both large-scale and regional-scale groundwater models have utility, strengths and limitations so continued modeling at both scales is essential and mutually beneficial. A crucial quest is how to evaluate the realism, capabilities and performance of large-scale groundwater models given their modeling purpose of addressing large-scale science or sustainability questions as well as limitations in data availability and commensurability. Evaluation should identify if, when or where large-scale models achieve their purpose or where opportunities for improvements exists so that such models better achieve their purpose. We suggest that reproducing the spatio-temporal details of regional-scale models and matching local data is not a relevant goal. Instead, it is important to decide on reasonable model expectations regarding when a large scale model is performing “well enough” in the context of its specific purpose. The decision of reasonable expectations is necessarily subjective even if the evaluation criteria is quantitative. Our objective is to provide recommendations for improving the evaluation of groundwater representation in continental- to global-scale models. We describe current modeling strategies and evaluation practices, and subsequently discuss the value of three evaluation strategies: 1) comparing model outputs with available observations of groundwater levels or other state or flux variables (observation-based evaluation); 2) comparing several models with each other with or without reference to actual observations (model-based evaluation); and 3) comparing model behavior with expert expectations of hydrologic behaviors in particular regions or at particular times (expert-based evaluation). Based on evolving practices in model evaluation as well as innovations in observations, machine learning and expert elicitation, we argue that combining observation-, model-, and expert-based model evaluation approaches, while accounting for commensurability issues, may significantly improve the realism of groundwater representation in large-scale models. Thus advancing our ability for quantification, understanding, and prediction of crucial Earth science and sustainability problems. We encourage greater community-level communication and cooperation on this quest, including among global hydrology and land surface modelers, local to regional hydrogeologists, and hydrologists focused on model development and evaluation.

Tom Gleeson et al.

Status: open (until 20 Jun 2021)

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Tom Gleeson et al.

Tom Gleeson et al.


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Short summary
Groundwater is increasingly being included in large-scale (continental to global) land surface and hydrologic simulations. However, it is challenging to evaluate these simulations because groundwater is “hidden” underground and thus hard to measure. We suggest using multiple complementary strategies to assess the performance of a model (“model evaluation”).