Trend analysis of reference evapotranspiration in Northwest China: The roles of changing wind speed and surface air temperature

Reference evapotranspiration (ET0) is an important element in the water cycle that integrates atmospheric demands and surface conditions, and analysis of changes in ET0 is of great significance for understanding climate change and its impacts on hydrology. As ET0 is an integrated effect of climate variables, increases in air temperature should lead to increases in ET0. However, this effect could be offset by decreases in vapor pressure deficit, wind speed, and solar radiation which lead to the decrease in ET0. In this study, trends in the Penman-Monteith ET0 at 80 meteorological stations during 1960-2010 in the driest region of China (Northwest China) were examined. The results show that there was a change point for ET0 series around the year 1993 based on the Pettitt's test. For the region average, ET0 decreased from 1960 to 1993 by -2.34mmyr(-2), while ET0 began to increase since 1994 by 4.80mmyr(-2). A differential equation method based on the Food and Agriculture Organization Penman-Monteith formula was used to attribute the change in ET0. The attribution results show that the significant decrease in wind speed dominated the change in ET0, which offset the effect of increasing air temperature and led to the decrease in ET0 from 1960 to 1993. However, wind speed began to increase, and the amplitude of increase in air temperature also rose significantly since the mid-1990s. Increases in air temperature and wind speed together reversed the trend in ET0 and led to the increase in ET0 since 1994. Copyright (c) 2012 John Wiley & Sons, Ltd.