After a year of correspondence, Miriam Coenders’ article was finally published in Nature last week. It explains why an earlier Nature publication on global water balance is incorrect.
‘Terrestrial water fluxes dominated by transpiration’, that is the title of the original article written by Scott Jasechko MSc, a PhD student at the University of New Mexico, dated April 18 2013, which kick started the discussion.
The American researcher asserts that 80 – 90% of the water that evaporates from land comes from plants. That is far larger than the reigning estimates. Coenders didn’t buy Jasechko’s figures. Instead of 80 – 90%, she arrives at 35 – 80% percent.
The relative amount of transpiration in the water cycle in comparison with evaporation from soil and open water is an interesting figure since on a global scale it links CO2 and water cycles.
“The high estimate has two reasons”, Coenders says. First there has been selective use of data known as cherry picking. Next, she argues the uncertainties in the data that Jascheko has used to calculate the transpiration are much bigger than he assumes. This applies in particular to the isotope studies. The basis of these studies is that transpiration keeps the ratio between oxygen isotopes O-16 and O-18 in water unchanged, but that evaporation favours the lighter isotopes and thus enriches the residues with the heavier isotope. Jasechko applied a lab-derived equation on evaporation to determine the effect and extrapolated his findings into global estimates.
On Coenders’ desk are Jasechko’s original article and three commentaries on the article written by her, Professor Huub Savenije (CEGS) and colleagues called Uncertainties in transpiration estimates. Nature editors have forwarded these to Jasechko, who in turn has written three reactions, which are on the desk as well. The final version of Coenders’ reaction to Jasechko’s article was published on February 12 2014, almost a year after the original. That’s how slow science sometimes progresses.
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