Evaporation from pans has been decreasing over many areas of the world for the past half century, but the significance of this trend is under debate.
Though some speculate that decreases in pan evaporation result from well-documented "global dimming," where less solar irradiance reaches the ground, others hypothesize a complementary relationship between pan evaporation and actual evaporation.
For example, in arid climates, terrestrial evaporation is low. However, water in pans left out in this environment can evaporate huge amounts of water. By contrast, water in pans left out in a more humid environment due to increased precipitation will tend to lose less water because of the ambient humidity.
In an article in a recent issue of the journal Geophysical Research Letters, Wilfried Brutsaert, of the School of Civil and Environmental Engineering, Cornell University, shows through a mathematical model how lower pan evaporation rates actually indicate higher terrestrial evaporation, in spite of global dimming.
Thus, while global dimming had an effect, it was not strong enough to cause a negative trend in evaporation where pan evaporation had been observed to decrease. Based on this, he suggests that the hydrologic cycle is accelerating in those areas.
Source: Geophysical Research Letters (GRL) paper 10.1029/2006GL027532, 2006
The above post is reprinted from materials provided by American Geophysical Union. Note: Content may be edited for style and length.
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