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The Water Cycle Research

University of Utah researchers determined how much of the rain and snowmelt that falls on the land moves to the atmosphere from plant transpiration and evaporation from soil and surface waters 

More than a quarter of the rain and snow that falls on continents reaches the oceans as runoff. Now a new study helps show where the rest goes: two-thirds of the remaining water is released by plants, more than a quarter lands on leaves and evaporates and what's left evaporates from soil and from lakes, rivers and streams.

"The question is, when rain falls on the landscape, where does it go?" says University of Utah geochemist Gabe Bowen, senior author of the study published today in the journal Science. "The  on the continents sustains all plant life, all agriculture, humans, aquatic ecosystems. But the breakdown - how much is used for those things - has always been unclear."

"Some previous estimates suggested that more water was used by plants than we find here," he adds. "It means either that plants are less productive globally than we thought, or plants are more efficient at using water than we assumed."

University of Utah hydrologist Stephen Good, the study's first author, says, "We've broken down the different possible pathways that water takes as it moves from rainfall [and snowfall] through soils, plants and rivers. Here we've found the proportions of water that returns to the atmosphere though plants, soils and open water."

The study used hydrogen isotope ratios of water in rain, rivers and the atmosphere from samples and satellite measurements to conclude that of all precipitation over land - excluding river runoff to the oceans—these amounts are released by other means:

  • 64 percent (55,000 cubic kilometers or 13,200 cubic miles) is released or essentially exhaled by plants, a process called transpiration. This is lower than estimated by recent research, which concluded plant transpiration accounted for more than 80 percent of water that falls on land and does not flow to the seas, Bowen says.
  • 6 percent (5,000 cubic kilometers or 1,200 cubic miles) evaporates from soils.
  • 3 percent (2,000 cubic kilometers or 480 cubic miles) evaporates from lakes, streams and rivers.
  • Previous research indicated the other 27 percent (23,000 cubic kilometers or 5,520 cubic miles) falls on leaves and evaporates, a process called interception.

"It's important to understand the amount of water that goes through each of these pathways," Good says. "The most important pathway is the water that passes through plants because it is directly related to the productivity of natural and agricultural plants."

In another key finding, the researchers showed how much rainwater or snowmelt passing through soils is available for plants to use before it enters groundwater, lakes or streams. They found this "connectivity" is 38 percent: Only 38 percent of water entering groundwater, lakes or rivers interacts with soil, and the rest "moves rapidly into groundwater and lakes and rivers without spending much time in the soil," Bowen says.

"Lot of things happen in soils: nutrients, fertilizers, contaminants, various biological processes," he adds. "If water that goes to streams and groundwater moves rapidly through soil, it has less interaction with those processes. It means the soils and rest of the hydrologic cycle are somewhat separated. If we want to predict future climate change, hydrologic change and water quality, we need to account for the fact that most water doesn't interact with soils before it reaches streams and groundwater."

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Source: Phys.org

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