Global water security relies on the origins of rainfall

Securing the world’s water supply is among the most pressing global challenges today. Researchers at Stockholm University have introduced an innovative method to better quantify the global risk of water scarcity. 

The findings suggest that the risks to the world’s water supply may be more significant than previously estimated when environmental conditions and governance in regions where rain-producing moisture originates are taken into account.

Understanding the global water supply 

Traditionally, the global water supply is understood as rain falling on the Earth’s surface, subsequently stored in aquifers, lakes, and rivers. This conventional understanding is often used to evaluate water security and the potential for water scarcity. 

However, a new study published in Nature Water challenges this perspective by highlighting the importance of governance and environmental conditions in regions upwind – where the moisture that eventually falls as rain is generated.

“Water supply really originates beforehand, with moisture evaporated from land or in the ocean traveling in the atmosphere before falling as rain. This upwind moisture is commonly overlooked when assessing water availability,” explained senior author Fernando Jaramillo, an associate professor in physical geography at Stockholm University.

Origins of rainfall

Typically, when a lake or river is shared between different countries or authorities, water assessments and regulations focus on upstream factors – considering the conditions upriver from the water body. 

The new study, however, adopts an upwind perspective, which takes into account the areas where evaporated water is transported before it precipitates as rain. This area, known as a “precipitationshed,” can span vast regions of the Earth’s surface.

“For instance, in tropical South America, most of the Amazon basin is downstream of the Andes mountain range, whereas large areas of the Andes are in themselves downwind of the Amazon rainforest and depend on it. This interdependency makes these two regions reliant on each other for water supply,” Jaramillo said.

Higher risks of water scarcity 

The researchers analyzed 379 hydrological basins globally, revealing that water security risks are considerably higher when the upwind origins of water are factored into the equation.

“With this approach, we see that 32,900 km³/year of water requirements worldwide face very high risk, a near 50 percent increase compared to the 20,500 km³/year resulting from the more traditional upstream focus,” said lead author José Posada, a former doctoral student at Stockholm University.

Political control and land use changes in upwind regions can have profound effects on downwind water availability. For example, deforestation and agricultural development in these areas can reduce the moisture provided by vegetation, subsequently decreasing rainfall downwind and heightening water security risks.

Water security across different regions

“For coastal countries such as the Philippines, most of the rain comes from the sea, so land-use changes pose minimal risk to their water security. Conversely, for inland countries like Niger, where rainfall is largely sourced from moisture that evaporates in neighboring countries such as Nigeria and Ghana, land-use changes in these upwind regions can significantly impact water security,” Jaramillo explained.

This implies that political factors, such as environmental management and regulatory policies in areas where moisture first evaporates, can greatly influence water safety in distant regions.

“The Congo River basin, for example, heavily relies on moisture from neighboring countries with low environmental performance and governance according to global indicators. This makes the basin vulnerable to risks from potential deforestation and unregulated land use changes in those neighboring areas,” said co-author Lan Wang-Erlandsson, a researcher at the Stockholm Resilience Center at Stockholm University.

All water is connected

The study underscores the importance of considering upwind perspectives in environmental regulation and governance. It highlights the interdependence between countries situated upstream/downwind and downstream/upwind.

“It is not possible to ignore the interdependence between countries. In the end, all water is connected, so we should not only mind how we manage our water resources within a region or country but also how our neighboring countries do,” Wang-Erlandsson said.

The researchers hope their findings will help identify key areas and stakeholders for cooperation strategies aimed at mitigating the causes of water-related tensions. This includes considering atmospheric water flows in transboundary decision-making and water governance frameworks.

“We stress the need for international cooperation to effectively manage upwind moisture sources,” Jaramillo concluded.

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