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

Gnann, S., Reinecke, R., Stein, L., Wada, Y. ORCID: https://orcid.org/0000-0003-4770-2539, Thiery, W., Müller Schmied, H., Satoh, Y., Pokhrel, Y., Ostberg, S., Koutroulis, A., Hanasaki, N., Grillakis, M., Gosling, S. N., Burek, P. ORCID: https://orcid.org/0000-0001-6390-8487, Bierkens, M.F.P., & Wagener, T. (2023). Functional relationships reveal differences in the water cycle representation of global water models. Nature Water 1 1079-1090. 10.1038/s44221-023-00160-y.

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Abstract

Global water models are increasingly used to understand past, present and future water cycles, but disagreements between simulated variables make model-based inferences uncertain. Although there is empirical evidence of different large-scale relationships in hydrology, these relationships are rarely considered in model evaluation. Here we evaluate global water models using functional relationships that capture the spatial co-variability of forcing variables (precipitation, net radiation) and key response variables (actual evapotranspiration, groundwater recharge, total runoff). Results show strong disagreement in both shape and strength of model-based functional relationships, especially for groundwater recharge. Empirical and theory-derived functional relationships show varying agreements with models, indicating that our process understanding is particularly uncertain for energy balance processes, groundwater recharge processes and in dry and/or cold regions. Functional relationships offer great potential for model evaluation and an opportunity for fundamental advances in global hydrology and Earth system research in general.

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