Even though we know that plants play a critical role in our planet’s water cycle, there are some striking gaps in that diagram we all first see as elementary school children. Our new paper in the journal Nature Water is the first to offer global estimates of the amount of water stored in vegetation on Earth’s surface and the time it takes for that water to flow through vegetation and return to the atmosphere.
We find that plants do not hold much water (786 cubic km) compared to other pools, but the time it takes for water to flow through plants and return to the atmosphere is exceptionally fast. It ranges from just 5 days in croplands to 18 days in evergreen needleleaf forests. In comparison, it’s estimated to take about 17 years for lake water to return to the atmosphere and 1600 years for water to return from glaciers.
To generate these estimates, we first estimated the amount of water stored in plants using data from NASA’s Soil Moisture Active Passive Mission (SMAP) satellite mission. The mission was launched to provide high resolution estimates of the water in soils. We realized that whereas the SMAP mission originally saw plants as interfering with their soil moisture measurements, and was correcting for their presence, those corrections actually held valuable information for understanding the water cycle. We then combined estimates of plant water storage with cutting-edge estimates of the rates at which water is leaving plants to ultimately determine the transit time of water through vegetation. The result was five years of monthly estimates of water storage and transit time at a spatial resolution of 9 km2.
We find that plants do not hold much water (786 cubic km) compared to other pools, but the time it takes for water to flow through plants and return to the atmosphere is exceptionally fast. It ranges from just 5 days in croplands to 18 days in evergreen needleleaf forests. In comparison, it’s estimated to take about 17 years for lake water to return to the atmosphere and 1600 years for water to return from glaciers.
To generate these estimates, we first estimated the amount of water stored in plants using data from NASA’s Soil Moisture Active Passive Mission (SMAP) satellite mission. The mission was launched to provide high resolution estimates of the water in soils. We realized that whereas the SMAP mission originally saw plants as interfering with their soil moisture measurements, and was correcting for their presence, those corrections actually held valuable information for understanding the water cycle. We then combined estimates of plant water storage with cutting-edge estimates of the rates at which water is leaving plants to ultimately determine the transit time of water through vegetation. The result was five years of monthly estimates of water storage and transit time at a spatial resolution of 9 km2.
In the images above, we show global variation in vegetation transit times within and among land- cover types including mean transit times (in days) of water through aboveground vegetation calculated on an annual basis. and violin and boxplots of mean transit times among land-cover types, with the horizontal line denoting the untruncated global median of 8.1 days (2.5 to 32.4, 5th and 95th quantiles). In sum, we found that the transit time of water through vegetation varied considerably across different land cover types and depending on climate at different times of year.
However, 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. Importantly, croplands around the world tend to have very similar and very fast transit times. 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.
Ours is a step forward in understanding how long it takes for a drop of rain to fall from the sky, move through the soil into the root of a plant, and return back to the sky. Without this information, we cannot estimate how the climate and land-use changes that we are imposing on our planet are accelerating and homogenizing our water cycle.
This study was nearly 13 years in the making (a story for another day!) and came to fruition because of the incredible work of Andrew Felton, PhD in collaboration with Joshua Fisher, Koen Hufkens, Lou Duloisy, Seth Spawn-Lee, and AJ Purdy.
This project was funded by United States Department of Agriculture (USDA) award number 2020- 67014-30917 to G.R.G. and NASA Research Opportunities in Space and Earth Science grant number 80NSSC20K0216 to G.R.G. and J.B.F. and number 80NSSC23K0309 to G.R.G. and J.B.F. Additional support to A.J.F. was provided by USDA award number 2021-67034-40252 and USDA Hatch project 7006859.
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