Mountains are the water towers of the world, supplying a substantial part of both natural and anthropogenic water demands1,2. They are highly sensitive and prone to climate change3,4, yet their importance and vulnerability have not been quantified at the global scale. Here we present a global water tower index (WTI), which ranks all water towers in terms of their water-supplying role and the downstream dependence of ecosystems and society. For each water tower, we assess its vulnerability related to water stress, governance, hydropolitical tension and future climatic and socio-economic changes. We conclude that the most important (highest WTI) water towers are also among the most vulnerable, and that climatic and socio-economic changes will affect them profoundly. This could negatively impact 1.9 billion people living in (0.3 billion) or directly downstream of (1.6 billion) mountainous areas. Immediate action is required to safeguard the future of the world’s most important and vulnerable water towers.
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The data generated to support the findings of this study are available in an online data repository at zenodo.org at https://doi.org/10.5281/zenodo.3521933. Third party data used in this study are available as follows. Hydrological basin boundaries5 used in this study are available online at http://www.fao.org/nr/water/aquamaps/. Mountain definition data6 used in this study are available online at https://ilias.unibe.ch/goto_ilias3_unibe_file_1047348.html. Precipitation and evaporation data used in this study32 are available online at https://cds.climate.copernicus.eu. Snow cover data used in this study7 are available online at https://nsidc.org/data/MOD10CM. Glacier volume data48 used in this study are available online at https://doi.org/10.3929/ethz-b-000315707. Glacier mass balance data17,49 are available online at https://wgms.ch/. Lake and reservoir storage data50 used in this study are available online at https://www.hydrosheds.org/pages/hydrolakes. Water demand data used in this study are available upon request from Y.W. (email@example.com). BWS data38 used in this study are available online at https://www.wri.org/aqueduct. GE data39 used in this study are available online at https://info.worldbank.org/governance/wgi/#home. Data on hydro-political tensions for transboundary river basins37 used in this study are available online at https://transboundarywaters.science.oregonstate.edu/content/transboundary-freshwater-spatial-database. Data for future projections of population count9 used in this study are available online at ftp://ftp.pbl.nl/hyde/SSPs/SSP2/zip/. Data for future projections of GDP41 used in this study are available online at http://www.cger.nies.go.jp/gcp/population-and-gdp.html. Data for future projections of temperature and precipitation40 used in this study are available online at https://climexp.knmi.nl. An online interactive visualization of the water tower index and vulnerability is available at https://www.nationalgeographic.com/environment/perpetual-planet/.
The code developed for the WTI calculations performed for this study are publicly available in a Github repository at https://github.com/mountainhydrology/pub_ngs-watertowers.
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This project was funded as part of the National Geographic Society and Rolex Partnership to Support a Perpetual Planet. We are grateful to the Strategic Priority Research Program of the Chinese Academy of Sciences for their support, to D. Farinotti for providing the data on glacier volume, and to N. Wanders for providing precipitation datasets used in the uncertainty analysis.
The authors declare no competing interests.
Peer review information Nature thanks Günther Grill and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.
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Extended data figures and tables
a, The WTUs are defined as the intersection of Earth’s major hydrological basins5 and mountain ranges6 meeting predefined thresholds for ice volume or snow persistence (see Methods section). One WTU can consist of (parts of) multiple mountain ranges and one mountain range can be part of multiple WTUs. The example shows two hydrological basins in North America: the Great Basin (red outline) and California (blue outline). The striped areas indicate two mountain ranges: the Sierra Nevada and the Cascade Range. The intersection of the hydrological basins and the mountain ranges defines the WTUs (dark colours). For example, the Great Basin WTU is defined as the portion of the Sierra Nevada that is part of the Great Basin hydrological basin (dark red), and the California WTU is defined as the portion of the Sierra Nevada that is part of the California hydrological basin as well as a portion of the Cascade Range that is part of the California hydrological basin (dark blue). The WTU’s dependent area (light colours) is defined as the sub-basins within the hydrological basin that are overlapping the WTU or downstream of sub-basins overlapping the WTU. b–e, The WTUs (dark colours) and associated WTU basins (light colours) for all 78 WTUs and WTU basins, grouped by continents: North America (b), Europe (c), Asia and Oceania (d), South America (e). Number labels indicate the WTU IDs (see Extended Data Tables 1, 2 for corresponding names).
a, The WTU SI (blue colourscale) and downstream DI (brown colourscale) for all 78 WTUs and WTU basins. b, Supply index (SI) and demand index (DI) for each WTU grouped per continent. Background colour gradient indicates water tower importance (that is, darker shades represent higher SI and DI values). Points are labelled with WTU IDs (see Extended Data Tables 1, 2, Extended Data Fig. 1).
Position change in ranking of WTUs by WTI resulting from uncertainty in input data (blue), expressed as a percentage of 1,000 realizations of the WTI index calculation. Position change in ranking of WTUs by WTI resulting from uncertainty in the weights of individual indicators (red), expressed as a percentage of 10,000 realizations of the WTI index calculation.
Excel table presenting an overview of all values for the supply indicators and subindicators, supply index, demand indicators, demand index, and Water Tower Index, for each Water Tower Unit. Explanations of all variables are provided in Methods and Extended Data Tables 3, 4.
Excel table presenting an overview of all values of vulnerability and future change indicators for each Water Tower Unit. Explanations of all variables are provided in Methods.
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Immerzeel, W.W., Lutz, A.F., Andrade, M. et al. Importance and vulnerability of the world’s water towers. Nature 577, 364–369 (2020). https://doi.org/10.1038/s41586-019-1822-y