A recent study offers the first extensive global assessments regarding the quantity of water held in Earth’s vegetation and the duration it takes for that water to move through these plants. This information is crucial for a deeper understanding of the global water cycle and the changes it undergoes due to shifts in land use and climate conditions.
A recent study headed by researchers from the Schmid College of Science and Technology at Chapman University delivers the inaugural thorough global assessments concerning the volume of water contained in Earth’s flora and the time frame required for that water to pass through them. This data fills a significant gap in comprehending the global water cycle and its alterations influenced by land use and climatic changes.
The study, which was published on January 9 in the journal Nature Water, reveals that Earth’s vegetation holds approximately 786 km3 of water, constituting merely about 0.002% of the Earth’s total freshwater reserves. Additionally, the research highlights that the duration for water to move through plants (known as transit or turnover time) and return to the atmosphere is among the quickest in the entire water cycle, varying from as little as five days in agricultural lands to 18 days in evergreen needleleaf forests. Water transit is notably swift in croplands, grasslands, and savannas. These findings emphasize the active role of vegetation in the water cycle. By comparison, the global average time for water to circulate through plants is 8.1 days, whereas the water in lakes is estimated to take 17 years, and glacial water around 1600 years.
“It has been known for quite some time that most water returning to the atmosphere from the ground does so via plants, but until now, we lacked precise knowledge of how long this transit takes. Our findings indicate that water flows through plants in a matter of days instead of months, years, or centuries, as happens in other segments of the water cycle,” stated the lead researcher, Dr. Andrew Felton, who conducted this work as a part of a U.S. Department of Agriculture Fellowship at Chapman University, and is currently a professor at Montana State University.
The team of researchers explains that by merging estimates of water transit through plants with the movement of water in the atmosphere (which takes about 8-10 days) and the time water spends in the soil before being absorbed by plants (approximately 60 to 90 days), they are beginning to estimate the overall duration for a drop of water to navigate through the terrestrial water cycle.
“Plants are often overlooked in discussions about the global water cycle,” remarked Felton. “In many instances, they are not even included in water cycle illustrations, which is ironic because we already recognize their vital role in transporting water from the ground back to the atmosphere.”
To achieve these estimates, the research team initially assessed the water held in plants utilizing data from NASA’s Soil Moisture Active Passive Mission (SMAP) satellite mission, which furnished high-resolution estimations of soil moisture. The SMAP mission previously regarded plants as a hindrance to soil moisture assessments and corrected for their impact. However, the Chapman investigators discovered that these corrections actually contained useful insights for understanding the water cycle. They integrated their findings on plant water storage with advanced estimates of how quickly water is released from plants to calculate the transit time through vegetation. The outcome was a comprehensive dataset covering five years of monthly estimates for water storage and transit time with a resolution of 9 km2.
The research team also identified significant variations in water transit times across various land cover types, climates, and seasons. Specifically, the time water spends passing through croplands was notably rapid and consistent, with water transiting through plants in under a day during peak growing seasons.
“A key observation is that croplands globally exhibit very similar and extremely quick transit times,” highlighted Dr. Gregory Goldsmith, the senior author and an associate professor at Chapman University. “This suggests that changes in land use could be standardizing the global water cycle and promoting its intensification by expediting the return of water to the atmosphere, leading to heavier rain events.”
“The findings imply that the transit duration of water through plants is likely very responsive to events such as deforestation, drought, and wildfires, which could significantly alter the time it takes for water to navigate through the water cycle,” Felton concluded.
