Emergent constraints in combination with multi-source observations greatly enhance the robustness of model-based assessments on the hydrological effect of land use and land cover changes (LULCC). It contributes to more accurate hydrological projections under the future afforestation scenario. This is the main result of a study led by Zefeng Chen and carried out with the participation of Giovanni Forzieri, both affiliated with the Department of Civil and Environmental Engineering at the University of Florence, Italy.
Anthropogenic LULCC has substantially affected the terrestrial water cycle during the past few decades, and its overall effect is believed to be of comparable magnitude to that one originating from climate change. Current state-of-the-art Earth system models (ESMs), which are used to frame climate strategies and define future scenarios that are widely used in climate-related policies, are typically exploited to quantify the long-term effect of LULCC on terrestrial water cycle at large scales. However, substantial discrepancies exist across ESMs due to their highly heterogeneous representation of key biological and physical processes, which have led to an ambiguous understanding of the hydrological response to land surface changes.
Using multiple observation-based products and a suite of CMIP6 ESMs, our study provides novel evidence that the model bias in representing the ratio between sensible heat and latent heat (represented by the transpiration-specific Bowen ratio) through transpiration results in inaccurate assessments of the LULCC effects on terrestrial evapotranspiration and water availability globally and regionally. By adjusting the transpiration-specific Bowen ratio to the observed value within the hierarchical emergent constraint approach, the sign of the original ESM estimates obtained at the global scale and over Central and South America, and narrows the inter-model spread, leading to more robust estimates of the LULCC-driven evapotranspiration (ET) change. The misrepresentation of transpiration-specific Bowen ratio and its variations across plant functional types in ESMs are the main sources of such substantial bias. By applying an analogous constraint framework to a future afforestation scenario, it shows that the constrained simulations project stronger ET enhancements and weaker decreases in terrestrial water availability induced by afforestation compared to the original ESM simulations, particularly in tropics and subtropics. This is likely due to the increase in precipitation driven by a higher ET and associated intensified moisture recycling.
All together these findings reveal the worrying performance of current-version ESMs on predicting the signal of LULCC effect on terrestrial water cycle, not only in terms of magnitude of the effect but also in terms of sign. This highlights the relevance of addressing model errors regarding surface energy partitioning to reduce uncertainties and biases in CMIP6 simulations. Given the strong water-carbon interplay in terrestrial ecosystems, our observationally constrained results may help the reduction of uncertainties in the prospect of implementing land-based climate mitigation policies, while supporting the development of more holistic ones.
Citation: Chen, Z., Cescatti, A., Xing, R. & Forzieri, G. Emergent constraints on the hydrological impacts of land use and land cover change. Nat Commun (2026).
https://www.nature.com/articles/s41467-026-69883-2