24 Jan 2024
 | 24 Jan 2024
Status: a revised version of this preprint is currently under review for the journal HESS.

Towards understanding the intrinsic variations of the Priestley-Taylor coefficient based on a theoretical derivation

Ziwei Liu, Hanbo Yang, Changming Li, and Taihua Wang

Abstract. Priestley-Taylor (PT) coefficient (α) is generally set as a constant value or fitted as an empirical function of environmental variables, and it can bias the evaporation estimation or hydrological projections. This study derives a theoretical equation for α using an atmospheric boundary layer model, which shows that α is a function of air temperature (T) and specific humidity (Q). More importantly, the derived expressions can well estimate the sensitivity of α to T and Q, that is, dα/dT and dα/dQ, compared to water surface observations. α is generally negatively associated with T and Q, and its changes are fundamentally controlled by T and modulated by Q. Based on climate model data, it is shown that the variation of α to T (negative association) is of great importance for long-term hydrological predictions. For practical and broad uses, a lookup graph is also provided to directly find the dα/dT and dα/dQ values. Overall, the derived expression gives a physically clear and straightforward approach to quantify changes in α, which is essential for PT-based hydrological simulation and projections.

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Ziwei Liu, Hanbo Yang, Changming Li, and Taihua Wang

Status: final response (author comments only)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • RC1: 'Comment on hess-2024-17', Anonymous Referee #1, 16 Feb 2024
  • RC2: 'Comment on hess-2024-17', Anonymous Referee #2, 09 Apr 2024
Ziwei Liu, Hanbo Yang, Changming Li, and Taihua Wang
Ziwei Liu, Hanbo Yang, Changming Li, and Taihua Wang


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Short summary
The determination of the coefficient (α) in the PT equation is always on the empirical side. Here based on an atmospheric boundary layer model, we derived a physically clear expression to investigate the behavior of α. We pointed out that the temperature dominates changes in α and emphasized that the variation of α to temperature should be well considered for long-term hydrological predictions. Our works advance and promote the most classical models in the field.