Research transforms our understanding of the terrestrial water cycle, with significant implications for climate and ecosystem health.
A new study from the Schmid College of Science and Technology at Chapman University offers the first comprehensive global estimates of how much water is stored in Earth’s plants and how quickly that water moves through them. This research fills a critical gap in understanding the global water cycle and how it is being affected by land use changes and climate change.
Published in Nature Water, the study reveals that Earth’s vegetation holds approximately 786 cubic kilometers of water—just 0.002% of the planet’s total freshwater supply. Additionally, the study shows that the time it takes for water to move through plants, known as transit or turnover time, is one of the fastest processes in the global water cycle. This duration ranges from as little as five days in croplands to 18 days in evergreen needleleaf forests.
The transit of water through plants is particularly fast in croplands, grasslands, and savannas. The results underscore vegetation’s dynamic role in the water cycle. In comparison to the global annual median of 8.1 days for water to transit through plants from entry to exit, the water in lakes is estimated to take 17 years, and the water in glaciers is estimated to take 1600 years.
Fast Water Movement Through Plants
“We have known for a long time that most of the water that returns from the ground to the atmosphere does so through plants, but until now, we did not really know how long it took for that water to transit through plants. Our results show that the transit of water through plants occurs on the order of days, rather than months, years, or centuries, as it does in other parts of the water cycle,” said the study’s lead author Dr. Andrew Felton, who carried out the work as part of a U.S. Department of Agriculture Fellowship while at Chapman University and is now a professor at Montana State University.
The research team notes that by combining estimates of the transit of water through plants with the transit of water through the atmosphere (about 8-10 days) and the transit time of water through soil before being taken up by plants (about 60 to 90 days), they can begin to estimate the complete amount of time it takes for a drop of water to move through the terrestrial water cycle.
Plants: The Overlooked Key in the Water Cycle
“Plants are the forgotten part of the global water cycle,” said Felton. “In many cases, plants are not even represented on water cycle diagrams, which is ironic because we already know they play this critical role in returning water from the ground to the atmosphere.”
To generate the estimates, the research team first calculated the amount of water stored in plants using data from NASA’s Soil Moisture Active Passive Mission (SMAP) satellite mission, which provided high-resolution estimates of the water in soils. The SMAP mission originally saw plants as interfering with the soil moisture measurements, and was correcting for their presence. The Chapman researchers found those corrections actually held valuable information for understanding the water cycle. The team combined estimates of plant water storage with cutting-edge estimates of the rates at which water is leaving plants to determine the transit time of water through vegetation. The result was five years of monthly water storage and transit time estimates at a spatial resolution of 9 km2.
The research team also found that the transit time of water through vegetation varied considerably across different land cover types, climate and seasons. The transit time of water through croplands was significantly and consistently the fastest, with water transiting through plants in less than a day during the peak of the growing season.
“One important observation is that croplands around the world tend to have very similar and very fast transit times,” said Dr. Gregory Goldsmith, senior author and an associate professor of Biological Sciences at Chapman University. “This indicates that land use change may be homogenizing the global water cycle and contributing to its intensification by more rapidly recycling water back to the atmosphere where it can turn into heavy rain events.”
“The results suggest that the transit time of water through plants is likely to be very sensitive to events such as deforestation, drought, and wildfire, which will fundamentally change the time it takes for water to flow through the water cycle,” Felton said.
Reference: “Global estimates of the storage and transit time of water through vegetation” by Andrew J. Felton, Joshua B. Fisher, Koen Hufkens, Adam J. Purdy, Seth A. Spawn-Lee, Lou F. Duloisy and Gregory R. Goldsmith, 9 January 2025, Nature Water.
DOI: 10.1038/s44221-024-00365-9
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