Working Group I: The Scientific Basis

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2.5.4 Evaporation

Only evaporation from the land surface is discussed, as nothing new since the SAR has emerged on oceanic evaporation changes. Land

The SAR reported widespread decreases of pan evaporation over the USA and Russia during the 20th century. Pan evaporation measurements are an index of evaporation from a surface with an unlimited supply of water (potential evaporation). Interpretation of this result involving potential evaporation as a decrease in actual land surface evaporation is contradictory to the temp-erature and precipitation increase reported in these areas, and the general intensification of the hydrological cycle over northern extra-tropical land areas (Brutsaert and Parlange, 1998). Further analysis by Lawrimore and Peterson (2000) supports Brutsaert and Parlange’s (1998) interpretation, as does Golubev et al. (2001). Using parallel observations of actual evaporation and pan evaporation at five Russian experimental sites, Golubev et al. (2001) developed a method to estimate actual land surface evaporation from the pan evaporation measurements. They showed that using this method, actual evaporation is shown to have increased during the second half of the 20th century over most dry regions of the United States and Russia. Similarly, over humid maritime regions of the eastern United States (and north-eastern Washington state) actual evaporation during the warm season was also found to increase. Only over the heavily forested regions of Russia and the northern United States did actual evaporation decrease. The increase in actual evaporation is related to the greater availability of moisture at the surface, due to increases in precipitation and the higher temperatures.

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