24 Oct 2022
24 Oct 2022
Status: this preprint is open for discussion.

What is the Priestley-Taylor Wet-Surface Evaporation Parameter? Testing Four Hypotheses

Richard Crago1, Joszef Szilagyi2,3, and Russell Qualls4 Richard Crago et al.
  • 1Department of Civil and Environmental Engineering, Bucknell University, Lewisburg, PA 17837, USA
  • 2Department of Hydraulic and Water Resources Engineering, Budapest University of Technology and Economics, Budapest, Hungary
  • 3Conservation and Survey Division, School of Natural Resources, University of Nebraska-Lincoln, Lincoln, NE, USA
  • 4Department of Biological Engineering, University of Idaho, Moscow, ID USA

Abstract. This study compares four different hypotheses regarding the nature of the Priestley-Taylor parameter α. They are: 1) α is a universal constant; 2) the Bowen ratio (H/LE, where H is the sensible and LE is the latent heat flux) for equilibrium (i.e. saturated air column near the surface) evaporation is a constant times the Bowen ratio at minimal advection (Andreas et al., 2013); 3) minimal advection over a wet surface corresponds to a particular relative humidity value, and 4) α is a constant fraction of the difference from the minimum value of one to the maximum value of α proposed by Priestley and Taylor (1972). Formulas for α are developed for the last three hypotheses. Weather, radiation and surface energy flux data from 171 FLUXNET eddy covariance stations were used. The condition LEref/LEp>0.90, was taken as the criterion for nearly-saturated conditions (where LEref is the reference and LEp is the apparent potential evaporation rate from the Penman (1948) equation). Daily and monthly average data from the sites were obtained. All formulations for α include one model parameter which is optimized such that the root mean square error of the target variable was minimized. For each model, separate optimizations were done for predictions of the target variables α, wet surface evaporation (α multiplied by equilibrium evaporation rate) and actual evaporation (the latter using a highly-successful version of the complementary relationship of evaporation). Overall, the second and fourth hypotheses received the best support from the data.

Richard Crago et al.

Status: open (until 19 Dec 2022)

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Richard Crago et al.


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Short summary
The Priestley-Taylor equation is widely used in hydrologic, climate and meteorological models to estimate evaporation. Alpha represents the impact of dry air that is carried into the region; this occurs even in extensive saturated regions. Four hypotheses regarding the nature of alpha are evaluated. Data from 171 FLUXNET stations were used to test the hypotheses. The best-supported hypothesis sees alpha as a constant fraction of the distance between theoretical minimum and maximum values.