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

Pan evaporation ( E p ) is an important indicator of water and energy and the decline of E p has been reported in many regions over the last decades. The climate and E p are dependent on each other. In this study, the temporal trends of E p and main E p drivers, namely mean air temperature ( T a ), wind speed ( u ), global solar radiation ( R s ), net long‐wave radiation( R n l ) 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 E p from 26 stations decreased with the rate of −11.91 mm a −2 ; the trends of R s , R n l , T a , 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 −1 and 9.523 Pa decade −1 , respectively. The diffuse irradiance is an essential parameter to model E p and quantify the contribution of climatic factors to changing E p . 60 724 observations of R s and diffuse solar irradiance ( R d ) from seven of the 26 stations were used to develop the correlation between the diffuse fraction ( R d / R s ), and the clearness index ( R s / R o ). On the basis of the estimation of the diffuse component of R s and climatic data, we modified the PenPan model to estimate Chinese micro‐pan evaporation ( E p ) and assess the attribution of E p dynamics using partial derivatives. The results showed that there was a good agreement between the observed and calculated daily E p values. The observed decrease in E p was 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 R n l had insignificant effect (0.35 mm a −2 ) on E p rates. The change of E p is the net result of all the climatic variables. Copyright © 2014 John Wiley & Sons, Ltd.