Spatial and temporal trend of potential evapotranspiration and related driving forces in Southwestern China, during 1961–2009

Potential evapotranspiration (ETo) is an integral component of the hydrological cycle and the global energy balance, and its long-term variation is of much concern in climate change studies. Trends of temporal and spatial patterns in potential evapotranspiration and related driving factors over southwestern China are evaluated for the period 1961–2009, based on data from 111 standard meteorological stations. The results showed that southwestern China is experiencing a statistical decrease of ETo at a rate of 5.0 mm/10 y during 1961–2009. This decrease mainly started in the 1980s when accelerated warming occurred, which reflected the existence of the “evaporation paradox”. However, autumn ETo showed a similar inter-annual variation with temperature rise. In spatial characteristics, the statistically significant decrease of ETo mainly occurred in the lower altitude region, whereas stations with increases or non-significant decreases are mainly in higher altitude regions. Through the analysis, the driving factors are the following: (1) the decreased wind speed is the main driving force for ETo decrease because the higher saturated vapor pressure would restrain evaporation owing to the weak water vapor exchange under lower wind speed. (2) Increased sunshine hours lead to stronger evaporation. This influence mainly occurred in the Xizang plateau–Hengduan Mountains and Yunnan–Guizhou plateau. (3) The weakly increased precipitation is also linked to the decreased ETo because the precipitation increase will cancel the partially increased ETo. (4) The decreased relative humidity would cause a corresponding decline of saturated vapor pressure, which must lead to the rise of ETo. (5) Climate warming is not the crucial driving factor for ETo inter-annual variation, but the accelerated warming had some influence on summer and autumn ETo, especially during the period of 1991–2009. In addition, the topography also had some influence. Thus, variation is dominated by the comprehensive functions of multi-factors, owing to the complex nature of ETo.