Upwind moisture supply increases risk to water security

Transboundary assessments of water security typically adopt an ‘upstream’ perspective, focusing on hazards and vulnerabilities occurring within a given hydrological basin. However, as the moisture that provides precipitation in the hydrological basin probably originates ‘upwind’, hazards and vulnerabilities potentially altering the moisture supply can be overlooked. Here we perform a global assessment of risk to water security in 379 hydrological basins accounting for upwind vulnerabilities and hazards from limited governance and environmental performance. We compare this upwind assessment with the more conventional approach focusing upstream. We find that accounting for upwind moisture supply increases the assessed risk to water security. The upwind perspective results in 32,900 km³ yr⁻¹ of water requirements (that is, the specific water needs of vegetation for their development) under very high risk, compared with 20,500 km³ yr⁻¹ under the upstream perspective. This study pinpoints the need to account for upwind moisture dependencies in global water-related risk assessments.