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