The transfer of water from rain and snow

15/09/2015

     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 water 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% (55,000 km3) 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% of water that falls on land and does not flow to the seas, Bowen says. 6% (5,000 km3) evaporates from soils and 3% (2,000 km3) evaporates from lakes, streams and rivers. Previous research indicated the other 27% (23,000km3) falls on leaves and evaporates, a process called interception.   Trần Tân