Variation Trend Analysis on Sunshine Duration and Its Impact Factors in Gannan Plateau in Recent 40 Years

Gannan Plateau located in the southwest of Gansu Province and in the northeast border region of the Tibetan Plateau,which is an important water conserving and supplying area in the upriver regions of Yellow River. It plays an important role in the maintenance of water resource and eco- environmental security of the Yellow River basin. Due to the impact of climate change and the man activities,the ecology and environment of Gannan Plateau had obviously changed. So,this paper carried out the research about the change characteristic of sunshine duration in Gannan Plateau in order to provide the reference for the study of climate change in the Tibetan Plateau.Based on the meteorological data of sunshine duration at 8 weather stations in Gannan Plateau from 1971 to 2000,the temporal and spatial variations of sunshine duration and its impact factors were studied by using trend analysis,Mann- Kendall test and principal component analysis,etc. The results show that the spatial distribution of annual average sunshine duration in the Gannan Plateau was far from uniformity. The sunshine duration in the western part of the Gannan Plateau was great more than that in the eastern part. The monthly variation of sunshine duration in the Gannan Plateau showed 3 peaks and 3 troughs. The sunshine duration in the 1970s and1980s was least,increased obviously in the 1990s and the sunshine duration in the western of the Gannan Plateau showed a decreasing tendency in the 2000s,while in the eastern it showed an contrary increasing tendency. Furthermore,the mutation analysis shows that there were 3 types of sunshine duration variety in the Gannan Plateau. The first type was the detected mutation,the sunshine duration was still increasing,such as Zhuoni and Diebu counties,the second was the detected mutation,but the sunshine duration showed a decreasing trend,such as Xiahe,Maqu and Zhouqu,and the third was no mutation,such as Hezuo,Luqu and Lintan. The variety of sunshine duration was closely related with the temperature,water vapor pressure,cloud amount,precipitation and wind speed,and the different variation tendency of precipitation and wind speed was the main effecting factors on the different variation tendency of sunshine duration in the eastern part and the western part of the Gannan Plateau.