|Malone, Robert - Rob|
Submitted to: Geophysical Research Letters
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/1/2001
Publication Date: N/A
Interpretive Summary: Research indicates that pan evaporation may be decreasing over Northern non-tropical land areas but, paradoxically, this is at odds with observed increases in temperature and precipitation. Pan evaporation is the quantity of water that evaporates from a large, non-enclosed (exposed to the atmosphere) pan over a specified period of time. It is fairly easy to measure and is important because it can be related to actual evapotranspiration (ET, combination of evaporation from soil and transpiration from plants) which is difficult to measure. There is little published information concerning whether decreases in pan evaporation can be interpreted as decreases in ET and under what conditions this relationship is parallel or opposite. Therefore, climatic zones where changes in ET and pan evaporation are parallel or opposite were investigated. Using this information, actual ET trends in the US and in the former Soviet Union were studied. In dry regions (ration of potential evaporation to precipitation greater than or equal to 0.8), the trend of pan evaporation and ET was observed to be opposite while in more moist regions the trend was parallel. Using pan evaporation data from various regions of the US, it was predicted that ET was increasing in most areas of the US with the possible exceptions of the Southeast and northern Great Lakes. Predicted ET trends now coincide with summer precipitation trends over much of the US and Russia. This research will assist people working in the area of global change to interpret how trends in pan evaporation data (much available data) relate to trends in ET (little available data). It also adds insight into the paradox of observed increases in precipitation and temperature and observed decreases in pan evaporation.
Technical Abstract: Observed decreases in pan evaporation over most of the United States and the former USSR during the post-WWII period, if interpreted as a decrease in actual evaporation, are at odds with increases in temperature and precipitation over Northern extratropical land areas. Using parallel observations of actual and pan evaporation at five Russian and Latvian experimental sites, we recalibrate trends in pan evaporation to make them more representative of actual evaporation changes. After applying this recalibration, pan evaporation time series over the dry regions of the United States and Russia reveal an increasing trend in actual evaporation during the past forty years. Over the Midwestern United States and humid maritime regions of the northeastern United States, actual evaporation in warm season was also found to increase but over the taiga zone of Russia and southeastern United States it decreases.