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

Abstract

Abstract Reference evapotranspiration ( ET 0 ) is an important element in the water cycle that integrates atmospheric demands and surface conditions, and analysis of changes in ET 0 is of great significance for understanding climate change and its impacts on hydrology. As ET 0 is an integrated effect of climate variables, increases in air temperature should lead to increases in ET 0 . However, this effect could be offset by decreases in vapor pressure deficit, wind speed, and solar radiation which lead to the decrease in ET 0 . In this study, trends in the Penman–Monteith ET 0 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 ET 0 series around the year 1993 based on the Pettitt's test. For the region average, ET 0 decreased from 1960 to 1993 by −2.34 mm yr −2 , while ET 0 began to increase since 1994 by 4.80 mm yr −2 . A differential equation method based on the Food and Agriculture Organization Penman–Monteith formula was used to attribute the change in ET 0 . The attribution results show that the significant decrease in wind speed dominated the change in ET 0 , which offset the effect of increasing air temperature and led to the decrease in ET 0 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 ET 0 and led to the increase in ET 0 since 1994. Copyright © 2012 John Wiley & Sons, Ltd.