Reference evapotranspiration trends and their sensitivity to climatic change on the Tibetan Plateau (1970-2009)

Evapotranspiration is an important component of the water and energy balance. It is dependent on climate. Precipitation, solar radiation, temperature, humidity, and wind all contribute to the rate of evapotranspiration. In this study, the temporal trends of reference evapotranspiration (ETref) and four main ETref drivers, namely, mean air temperature (T-a), wind speed (u(2)), net radiation (R-n) and actual vapour pressure (e(a)) from 1970 to 2009, were calculated based on 75 meteorological stations on the Tibetan Plateau. The results showed that the ETref on the Tibetan Plateau decreased on average by 0.6909mm a(-1)a(-1) from 1970 to 2009. T-a and e(a) showed an increasing trend, whereas u(2) and R-n exhibited a decreasing trend. To explore the underlying causes of the ETref variation, an attribution analysis was performed to quantify the contribution of T-a, u(2), R-n and e(a), which showed that the changes in u(2), R-n and e(a) produced the negative effect, whereas T-a produced the positive effect on ETref rates. The changes in u(2) were found to produce the largest decrease (-0.7mm) in ETref, followed by e(a) (-0.4mm) and R-n (-0.1mm). Although the significant increase in T-a had a large positive effect (0.51mm) on ETref rates, changes in the other three variables each reduced ETref rates, resulting in an overall negative trend in ETref. Copyright (c) 2012 John Wiley & Sons, Ltd.