Evolution regularity of precipitation in the Shiyang River Basin on Mann-Kendall features.

The spatio-temporal characteristics of precipitation were analyzed in the Shiyang River Basin,Gansu Province,China,on the basis of monthly precipitation dada collected at 5 meteorological stations from their foundation to 2008 and by means of Mann-Kendall method.The results show that the variation in precipitation is characterized by regional difference.It means that the precipitation in upper reaches decreases obviously(Zc=-0.561),a little increases in the middle reaches and a little decreases in the lower reaches.The annual precipitation is with wave-shaped variation,and during last 10 years the phenomena of air moisture increasing was obvious and the meteorological change tended to warming-wetting.The annual precipitation was not even extremely with unstable precipitation in August to the utmost and obvious differential between seasonal precipitations.In addition,there were quasi-17 years' and quasi-9 years' periodical vibration for upper reaches precipitation and lower reaches precipitation,respectively with the middle reaches' precipitation having this kind of two main periodical variations.As for spatial distribution of precipitation,it gradually decreases from south to north,resulting in following precipitation ranking in quantity: Wushao Ridge's precipitation Gulang's Yongchang's Wuwei's Minqin's.