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On the theory relating changes in area‐average and pan evaporation
Author(s) -
Shuttleworth W. James,
SerratCapdevila Aleix,
Roderick Michael L.,
Scott Russell L.
Publication year - 2009
Publication title -
quarterly journal of the royal meteorological society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.744
H-Index - 143
eISSN - 1477-870X
pISSN - 0035-9009
DOI - 10.1002/qj.434
Subject(s) - evaporation , planetary boundary layer , atmosphere (unit) , potential evaporation , environmental science , atmospheric sciences , coupling (piping) , boundary layer , meteorology , materials science , mechanics , geology , physics , metallurgy
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