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Title: ON THE THEORY RELATING CHANGES IN AREA-AVERAGE AND PAN EVAPORATION

Author
item SHUTTLEWORTH, W. - UNIVERSITY OF ARIZONA
item SERRAT-CAPDEVILA, A. - UNIVERSITY OF ARIZONA
item RODERICK, M. - AUSTRALIAN NAT'L. UNIV.
item Scott, Russell - Russ

Submitted to: Quarterly Journal Royal Meteorological Society
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2009
Publication Date: 7/10/2009
Citation: Shuttleworth, W.J., Serrat-Capdevila, A., Roderick, M.L., Scott, R.L. 2009. ON THE THEORY RELATING CHANGES IN AREA-AVERAGE AND PAN EVAPORATION. Quarterly Journal Royal Meteorological Society. 135: 1230-1247. doi: 10.1002/qj.434

Interpretive Summary: Actual evaporation can be measured either by using sophisticated meteorological techniques or by measuring the mass of liquid water loss from representative sample volumes. However, all of the available methods are either too recent and/or too inaccurate for them to be used effectively in analyses of small, long-term change in actual evaporation. Consequently, there is little or no direct plot-scale data on actual evaporation with sufficient longevity and accuracy to allow them to be used to investigate past changes in actual evaporation. This paper reconsiders the theory relating gradual change in the rate of actual evaporation from the natural landscape with focus on the relationship between this rate and the actual rates of evaporation from open water or evaporation pans. The resulting theory dictates that changes in the area-average evaporation from a landscape with changes in the evaporation from pans or open water can arise in by processes related to large-scale changes in atmospheric concentrations and circulation that modify surface evaporation rates in the same direction and by processes related to the coupling between the surface and atmosphere at the landscape scale that usually modify area-average evaporation and pan evaporation in different directions.

Technical Abstract: Theory relating changes in the area-average evaporation from a landscape with changes in the evaporation from pans or open water within the landscape is developed. Such changes can arise in two ways, by Type (a) processes related to large-scale changes in atmospheric concentrations and circulation that modify surface evaporation rates in the same direction, and by Type (b) processes related to coupling between the surface and ABL at landscape scale that usually modify area-average evaporation and pan evaporation in different directions. The theoretical basis for interrelating evaporation rates in response to Type (a) changes in climate is derived. They have the same sign, and they have broadly similar magnitude except for area-average evaporation which responds to area-average surface resistance. The results of previous modeling studies that investigate surface-atmosphere coupling are parameterized and, as an alternative to assuming the Complementary Evaporation hypothesis, used to develop a theoretical description of Type (b) coupling via the vapor pressure deficit in the ABL. This theory shows that the interrelationship between appropriately normalized pan and area-average evaporation rates varies significantly with temperature and wind speed but on average changes are approximately equal and opposite. Long-term Australian pan evaporation data are then analyzed to demonstrate the simultaneous presence of Type (a) and (b) processes, and observations from three field sites in southern Arizona are analyzed to confirm the theory describing Type (b) coupling via vapor pressure deficit.