A new Israeli study challenges a long-standing assumption about how ecosystems lose water to the atmosphere, suggesting that even small changes in rainfall may have a greater impact on water availability than previously thought, according to TPS-IL.

A research team at the Weizmann Institute of Science found that plant transpiration — the release of water vapor during photosynthesis — appears to operate with a relatively stable upper limit across different climates and vegetation types. This contrasts with the common assumption used in many climate and hydrological models that transpiration rises and falls proportionally with rainfall and environmental conditions.

As a result, ecosystems may continue losing water to the atmosphere at close to this upper threshold even when rainfall decreases. This leaves less water available as “water yield” — the portion of rainfall remaining after losses from evaporation and transpiration — for rivers, groundwater recharge, agriculture, and human consumption.

The study, published in Nature Communications, suggests that relatively small declines in rainfall could therefore translate into disproportionately large reductions in usable water resources.

Researchers also developed an index to better estimate ecosystem water availability by accounting for a largely fixed level of atmospheric water loss, rather than relying on rainfall totals alone.

“The use of this index shows that ecosystems in arid regions, like Israel, are more sensitive than we thought to climate change and are closer to their survival limit,” said Prof. Dan Yakir of the Weizmann Institute, an Israel Prize laureate in Earth Sciences who participated in the study. “Humid regions, on the other hand, are more sensitive to flooding.”

The research team, led by Dr. Eyal Rotenberg, based its findings on long-term data from FLUXNET, a global network of monitoring stations that has tracked exchanges of carbon dioxide, water, and energy between land ecosystems and the atmosphere since the 1990s. The study also used climate model projections to assess how water availability may change under different scenarios.

According to the researchers, more than 60% of rainfall over land returns to the atmosphere through evaporation and transpiration, a figure that can approach 100% in arid regions.

The authors say their findings have implications for understanding future water security risks, particularly in dry regions that may be more vulnerable to reduced rainfall than previously estimated, while wetter regions could face increased flood risks as more precipitation remains within the system.

They suggest the new index could help improve drought forecasting, water resource management, and agricultural planning, as well as support more refined climate modelling and infrastructure planning, including reservoirs, drainage systems, and desalination capacity.