New Metric Challenges Understanding of Global Water Cycle
– A new metric developed by scientists at the Weizmann Institute of Science is prompting a re-evaluation of how the global water cycle functions, with potentially significant consequences for regions already facing water scarcity and the escalating impacts of climate change.
Published in Nature Communications, the research suggests that the amount of water evaporating from land – a process known as evapotranspiration – has a surprisingly fixed upper limit. This finding implies that even relatively small decreases in rainfall could lead to disproportionately large reductions in available water resources, a scenario that could exacerbate existing water challenges worldwide.
For decades, scientists have largely viewed the global water cycle as a simple balance between “income” – precipitation – and “expenditure” – evaporation. However, the work led by Dr. Eyal Rotenberg, a staff scientist, and Professor Dan Yakir, an Israel Prize laureate, indicates that this “expenditure” side of the equation is more constrained than previously understood.
“Understanding how water moves through the environment is crucial as climate change intensifies,” explained the research team. The study highlights the critical role of evapotranspiration, particularly through vegetation, and demonstrates that this process is more rigid than previously thought. This rigidity suggests that ecosystems, especially those in arid regions, may be closer to their survival thresholds than previously believed.
The Limits of Evaporation
The research reveals that evapotranspiration remains relatively constant regardless of climate or vegetation type, establishing a stable upper limit. This contradicts long-held assumptions and raises concerns about the vulnerability of ecosystems to even minor shifts in rainfall patterns.
This discovery comes at a time when the world is witnessing increasingly frequent and intense hydrological extremes. A visualization released by NASA on , based on data from the GRACE and GRACE-FO satellites, shows a marked increase in both droughts and pluvials – periods of excessive precipitation – over the twenty-year period from 2002 to 2023. The visualization demonstrates that the intensity of these extreme events has risen in correlation with increasing global temperatures.
The NASA data, which analyzed 1,138 extreme wet and dry events, illustrates a growing trend of hydrological volatility. The volume of the spheres representing these events in the visualization is proportional to their intensity, measured in cubic kilometer months.
Hydrological Whiplash and Future Projections
The implications of this new understanding extend to the concept of “hydrological whiplash” – rapid shifts between drought and flood. A recent study published in Nature assesses historical extremes and future projections using a normalized streamflow metric, revealing sharp regional disparities in the U.S. The study found that drought deficits have, in some areas, exceeded 300% of normal flow during multi-year droughts.
Researchers linked these hydrologic outcomes with the Federal Emergency Management Agency’s National Risk Index, discovering that counties experiencing the most severe droughts also face the highest annual economic losses and the lowest resilience. Projections under a high-emissions scenario suggest that sustained surpluses will only emerge in regions already considered resilient, further highlighting the vulnerability of other areas.
The findings underscore the urgent need for region-specific and adaptive water management strategies to protect vulnerable populations from worsening climate extremes. The new metric developed by the Weizmann Institute scientists offers a fresh perspective on water management and climate resilience, potentially enabling more effective strategies for navigating a future defined by increasing hydrological uncertainty.
The research offers a new lens through which to view water management and climate resilience, emphasizing the importance of understanding the limits of evaporation in a changing climate.
