Analysis of the Spatiotemporal Distribution of Precipitation in Zhangye City Using Mann-Kendall Method

Heihe River locates in the arid region of northwest China.In the last five decades,the climate in Heihe River Basin changed significantly.Zhangye City lies in the middle reaches of Heihe River with little rainfall and its average annual precipitation is 198mm.Precipitation is not only one of main sources of water resources,but also one of the main factors affecting the periodic change of water resources over this area.According to the precipitation data of six typical meteorological stations from 1956 to 2004,this paper analyzed the changing characteristics of precipitation in Zhangye City by using Mann-Kendall rank analysis method to finish examination of annual and seasonal precipitation trends in Zhangye City.The Mann-Kendall statistic test is a rank-based and non-parametric approach which has been widely used to estimate the significance of long-term trends in hydro-meteorological time series.The results showed that the change of precipitation fluctuated in the last 49 years.The minimum and maximum precipitation happened in 1962 and 1979,which were 122mm and 289mm respectively.The precipitation of the 1990s was higher than that in the 1960s.The change of precipitation synchronized with temperature change. In other words,if precipitation increases,the temperature would be higher,and if precipitation decreases,the temperature would become lower.The general tendency of annual runoff kept fluctuating while the mean annual precipitation tended to increase(β=4.7mm/a),especially in autumn(β=3.9mm/a).The precipitation in summer and spring showed a declining trend,however, the precipitation in summer was 51.9%of the annual precipitation.Spatial distribution of precipitation showed that there was a gradual decreasing trend from southeast to northwest,with the elevation increasing precipitation gradually increasing.Topographic differentiation has led to such kind of spatial distribution of precipitation.Seasonal rainfall varied in different regions,and there was also an obvious increasing center in Minle District.The precipitation of Gaotai District and Linze District in summer tended to decline and so did the precipitation of Ganzhou District in spring(β=-5.1 mm/a).There was an obvious increasing tendency in the autumn precipitation of Shandan District(β=4.2 mm/a),and so did the winter precipitation of Minle District(β=0.65 mm/ a).In the last part,some problems need solving in the future were briefly analyzed.