“How much rain fell?” is a key question in any discussion about climate. But perhaps there is an even more important one. Like any household budget, the global water economy is based on “income,” that is, water entering the system as precipitation, and “expenditure” – water leaving the system through various forms of evaporation. On land, water evaporates mainly through vegetation, in a process known as evapo-transpiration.

In a study recently published in Nature Communications, scientists at the Weizmann Institute of Science found that, contrary to previous assumptions, evapo-transpiration has a stable upper limit, remaining constant under different climate and vegetation conditions.

The discovery that expenditure lacks flexibility has major implications for the global water cycle. It means that even a relatively small shift in rainfall, for example, as a result of climate change, could translate into disproportionate changes in “water yield,” meaning the difference between the water entering the system and the water lost to evaporation. In other words, arid regions may lose available water sources much faster, while wetter regions could face an increasing risk of flooding and flash floods.

The research team, led by Dr. Eyal Rotenberg, a staff scientist in the group of Israel Prize laureate Prof. Dan Yakir, based their study on projections from climate models and on long-term data from FLUXNET – a global network of measurement stations at hundreds of sites worldwide that has monitored exchanges of carbon (CO₂), water and energy between terrestrial ecosystems and the atmosphere since the 1990s. Their findings challenge prevailing assumptions in the field and suggest that when it comes to understanding the impacts of climate change on ecosystems and water resources, changes in water availability are a more meaningful metric than precipitation alone.