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

Abstract

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.

Spatiotemporal trends of reference evapotranspiration and its driving factors in the Beijing–Tianjin Sand Source Control Project Region, China

“…ET O variations can arise from coupling between the surface and atmospheric boundary layer at the landscape scale (Shuttleworth et al, 2009). Irrigation is one important man-made process that contributes to interactions between the land surface and atmosphere (Han et al, 2014).…”

A generalized complementary relationship between actual and potential evaporation defined by a reference surface temperature

“…In other words, if the surface resistance was truly zero then the measured meteorology would have been different. One way to address the coupling between surface resistance and meteorological variables using the Penman-Monteith approach equation is to introduce additional resistance(s) such as the climatological resistance [Shuttleworth et al, 2009]. That represents a useful approach but here we sought to further investigate the CR using equation (1) as a basis.…”

Evaluating anemometer drift: A statistical approach to correct biases in wind speed measurement

“…∼ 10-meter height) wind speed has declined on average − 0.140 meter per second per decade (m s − 1 dec − 1 ) over continental surfaces in the last 30-50 years (McVicar et al, 2012); a phenomenon termed "stilling" for the first time by Roderick et al (2007). The drivers behind this weakening in observed wind speed have been partly attributed to: (i) changes in land surface friction force because of forest growth, urbanization and other land use changes (Vautard et al, 2010;Bichet et al, 2012;Wever, 2012); (ii) decadal variability of atmospheric circulation (Lu et al, 2007;Azorin-Molina et al, 2014; (iii) increase of aerosol emissions and greenhouse gas concentrations (Jacobson and Kaufman, 2006;Xu et al, 2006); (iv) decrease of the spatial variance in both atmospheric pressure and air temperature (Kim and Paik, 2015); (v) positive trends in available soil water (Shuttleworth et al, 2009); (vi) astronomical changes (Mazzarella, 2007); and (vii) instrumental issues including technological improvements of wind sensors, maintenance and calibration issues, shifts in measurement sites, and time intervals at which data is stored (Wan et al, 2010;Azorin-Molina et al, 2017a). However, the causes of stilling remains uncertain as all these issues are likely occurring simultaneously with varying spatio-temporal variance.…”