More than half of the solar energy that reaches the Earth and is absorbed by land srfaces is used for evaporation of water. Evapotranspiration is a joint process of moisture evaporation from the land and transpiration from plants. Recently a survey (Nature 2010, 467, 951) was completed taht analyzed and generalized data from 253 globally distributed water vapor flux measurement and monitoring stations over a time span of 1982 to 2008. The results indicate that during 1982 to 1997 there has been an increase of the global annual evapotranspiration by 7.1 ± 1.0 mm. After that period increase of the global evapotranspiration seems to have ceased. The prime reason for this is the decrease of the average global soil moisture level.

A question of utmost importance is whether these changes reflect the natural climate variability or indicate a trend in the global water cycle, possibly being a consequence of a global climate change.

The key to answering this question is the quality of the measurement data provided by the monitoring stations. Two aspects are critical: (1) The temporal stability of the measurement systems, so that measurement data obtained over a long time period would be comparable and (2) comparability of data between stations operating in different locations around the globe.

(Image: Wikipedia)
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