Extended Data Fig. 1: Concept and global spread of WTUs.
From: Importance and vulnerability of the world’s water towers
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).
