Reference evapotranspiration change and the causes across the Yellow River Basin during 1957-2008 and their spatial and seasonal differences

摘要

As an indicator of atmospheric evaporating capability over a hypothetical reference surface, reference evapotranspiration (ET_0) is the most important hydrological and meteorological variable to reflect climate change. This is particularly true for the Yellow River Basin, which faces serious water shortages and is vulnerable to climate change. In this study, the ET_0 at 80 sites during 1957-2008 in the Yellow River Basin was calculated using the Penman-Monteith method with the calibrated Angstrom coefficients. Spatial and seasonal patterns of changes in ET_0 as well as the concerned climatic variables are specially focused on using advanced statistical tests and GIS method. The entire Yellow River Basin is characterized by complicated spatial variability in the change of ET_0. Significant negative trends are mainly distributed in the southeast corner, northern side, and midwest of the Yellow River Basin, while significant increases of ET_0 mainly occur in the middle part and southwest corner of the basin. Still, no coherent spatial patterns in ET_0 trends are seen in any season. The dominance of warming trends in temperature and decreasing trends in wind speed and sunshine duration can be found in the basin. Relative humidity presents insignificant or weak trends at many sites but with a mixed spatial structure of positive and negative trends at both annual and seasonal scales. The combined effects of climatic variables to ET_0 changes and their spatial and seasonal variability are revealed by further analysis of sensitivity of ET_0 to climatic variables and the contribution of climatic variables to ET_0 changes over six homogenous regions identified by a rotated empirical orthogonal function (REOF) clustering method on annual and seasonal scales. The decline of surface wind speed offsets the increasing effect of the temperature increase and is mainly responsible for the ET_0 reduction in the west and north of the Loess Plateau. The reduced sunshine duration is the leading factor for ET_0 decrease in the middle-lower Yellow River Plain, especially during the summer time. The increasing mean temperature plays the most important role in the ET_0 increase in the source area of the Yellow River Basin. Furthermore, regional actual evapotranspiration and ET_0 present complementary behavior, but does not accurately fall in the 1:1 complementary relationship of the Bouchet's hypothesis, especially for the high elevation subregions. In addition, although precipitation changes are the main driving factors for drought variation, increasing ET_0 intensified the drought in middle regions.