Pan evaporation modelling and changing attribution analysis on the Tibetan Plateau (1970–2012)

• Published in: Hydrological processes 2015-04, Vol.29 (9), p.2164-2177
• Main Authors: Xie, Hong, Zhu, Xuan, Yuan, Dao‐Yang
• Format: Article
• Language: English
• Subjects: Air temperature; Clearness index; Climate change; climatic change; Climatic data; climatic factors; Diffusion; energy; Evaporation; hydrology; Irradiance; light intensity; Mathematical models; Pan evaporation; Radiation; Solar irradiance; Solar radiation; Stations; Tantalum; Temperature effects; Tibetan Plateau; Trends; Vapor pressure; Vapour pressure; Weather stations; Wind; Wind speed
• ISSN: 0885-6087; 1099-1085
• DOI: 10.1002/hyp.10356

Pan evaporation (Eₚ) is an important indicator of water and energy and the decline of Eₚhas been reported in many regions over the last decades. The climate and Eₚare dependent on each other. In this study, the temporal trends of Eₚand main Eₚdrivers, namely mean air temperature (Tₐ), wind speed (u), global solar radiation (Rₛ), net long‐wave radiation(Rₙₗ) and vapour pressure deficit (D) from 1970 to 2012, were calculated on the basis of 26 meteorological stations on the Tibetan Plateau. The arithmetic average of Eₚfrom 26 stations decreased with the rate of −11.91 mm a⁻²; the trends of Rₛ, Rₙₗ, Tₐ, u and D were −1.434 w m⁻² decade⁻¹, 0.2511 w m⁻² decade⁻¹, 0.3590°C decade⁻¹, −0.2376 m s⁻¹ decade⁻¹and 9.523 Pa decade⁻¹, respectively. The diffuse irradiance is an essential parameter to model Eₚand quantify the contribution of climatic factors to changing Eₚ. 60 724 observations of Rₛand diffuse solar irradiance (Rd) from seven of the 26 stations were used to develop the correlation between the diffuse fraction (Rd/Rₛ), and the clearness index (Rₛ/Rₒ). On the basis of the estimation of the diffuse component of Rₛand climatic data, we modified the PenPan model to estimate Chinese micro‐pan evaporation (Eₚ) and assess the attribution of Eₚdynamics using partial derivatives. The results showed that there was a good agreement between the observed and calculated daily Eₚvalues. The observed decrease in Eₚwas mostly due to declining wind speed (−13.7 mm a⁻²) with some contributions from decreasing solar irradiance (−3.1 mm a⁻²); and the increase of temperature had a large positive effect (4.55 mm a⁻²) in total whilst the increase of Rₙₗhad insignificant effect (0.35 mm a⁻²) on Eₚrates. The change of Eₚis the net result of all the climatic variables. Copyright © 2014 John Wiley & Sons, Ltd.