On the theory relating changes in area‐average and pan evaporation

Theory relating changes in area‐average evaporation with changes in the evaporation from pans or open water is developed. Such changes can arise by Type (a) processes related to large‐scale changes in atmospheric concentrations and circulation that modify surface evaporation rates in the same direct...

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Bibliographic Details
Published in:Quarterly journal of the Royal Meteorological Society 2009-07, Vol.135 (642), p.1230-1247
Main Authors: Shuttleworth, W. James, Serrat‐Capdevila, Aleix, Roderick, Michael L., Scott, Russell L.
Format: Article
Language:English
Subjects:
Atmospheric boundary layer
climate change
complementary evaporation
Earth, ocean, space
Evaporation
evaporation paradox
Evaporation rate
Exact sciences and technology
External geophysics
global dimming
Joining
Landscapes
Meteorology
Physics of the high neutral atmosphere
Surface resistance
Vapor pressure
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Summary:Theory relating changes in area‐average evaporation with changes in the evaporation from pans or open water is developed. Such changes can arise by Type (a) processes related to large‐scale changes in atmospheric concentrations and circulation that modify surface evaporation rates in the same direction, and Type (b) processes related to coupling between the surface and atmospheric boundary layer (ABL) at the landscape scale that usually modify area‐average evaporation and pan evaporation in different directions. The interrelationship between evaporation rates in response to Type (a) changes is derived. They have the same sign and broadly similar magnitude but the change in area‐average evaporation is modified by surface resistance. As an alternative to assuming the complementary evaporation hypothesis, the results of previous modelling studies that investigated surface–atmosphere coupling are parametrized and used to develop a theoretical description of Type (b) coupling via vapour pressure deficit (VPD) in the ABL. The interrelationship between appropriately normalized pan and area‐average evaporation rates is shown to vary with temperature and wind speed but, on average, the Type (b) changes are approximately equal and opposite. Long‐term Australian pan evaporation data are analyzed to demonstrate the simultaneous presence of Type (a) and (b) processes, and observations from three field sites in southwestern USA show support for the theory describing Type (b) coupling via VPD. Copyright © 2009 Royal Meteorological Society
ISSN:0035-9009
1477-870X
1477-870X
DOI:10.1002/qj.434