Changes in reference evapotranspiration and its driving factors in the middle reaches of Yellow River Basin, China.

Reference evapotranspiration (ET) is important for agricultural, environmental and other studies, and understanding the attribution of its change is helpful to provide information for irrigation scheduling and water resources management. The present study investigates the attribution of the change of ET at 49 meteorological stations in the middle reaches of Yellow River basin (MRYRB) of China from 1960 to 2012. Results show that annual ET increases from the northwest to the southeast of MRYRB in space. We find that annual ET clearly presents a zigzag change pattern rather than a monotonically change during the whole period. The detected three breakpoints at 1972, 1988 and 1997 divide the whole period into four subperiods. The sensitivity analysis indicates that the ET is the most sensitive to surface solar radiation (Rs), followed by relative humidity (RH) and mean air temperature (T), and the least sensitive to wind speed (u) in our study area. Furthermore, we find that ET is becoming less sensitive to RH and more sensitive to T during 1960-2012. The attributions of the change in ET vary largely at different regions and subperiods. The declined wind speed is the dominant factor, followed by Rs to the ET reduction during 1960-2012. Further analysis shows that Rs and u are the two major contributing factors that control the change of ET at most stations and during most subperiods. Our study confirms that the change of ET is influenced by the complex interactions of climatic factors, and the dominant factor to the change of ET is different in various regions and time periods. The results presented here can provide a reference for agricultural production and water resources management in MRYRB as well as other semi-arid and semi-humid regions.