Attributing the Changes in Reference Evapotranspiration in Southwestern China Using a New Separation Method

This study investigated monthly and annual reference evapotranspiration ETₒ changes over southwestern China (SWC) from 1960 to 2012, using the Food and Agriculture Organization of the United Nations’ report 56 (FAO-56) Penman–Monteith equation and routine meteorological observations at 269 weather sites. During 1960–2012, the monthly and annual ETₒ decreased at most sites. Moreover, the SWC regional average trend in annual ETₒ was significantly negative (p < 0.05); this trend was the same in most months. A new separation method using several numerical experiments was proposed to quantify each driving factor's contribution to ETₒ changes and exhibited higher accuracy based on several validation criteria, after which an attribution analysis was performed. Across SWC, the declining annual ETₒ was mainly due to decreased net radiation (RN). Spatially, the annual ETₒ changes at most sites in eastern SWC (excluding southeastern West Guangxi) were generally due to RN, whereas wind speed (WND) or vapor pressure deficit (VPD) was the determinant at other sites. Nevertheless, the determinants differed among 12 months. For the whole SWC, increased VPD in February and decreased WND in April, May, and October were the determinant of decreased ETₒ; however, decreased RN was the determinant in other months. Overall, the determinant of the monthly ETₒ changes exhibited a complex spatial pattern. A complete analysis of ETₒ changes and the related physical mechanisms in SWC is necessary to better understand hydroclimatological extremes (e.g., droughts) and to develop appropriate strategies to sustain regional development (e.g., water resources and agriculture). Importantly, this separation method provides new perspective for quantitative attribution analyses and thus may be implemented in various scientific fields (e.g., climatology and hydrology).