Research On The Temporal And Spatial Features Of Extreme Precipitation Event In Xiangjiang River Basin In Recent 48 Years

Under the background of global warming, frequent extreme weather events have become a major concern of the international community. Exploring the spatial,temporal distribution of regional extreme rainfall events and the causes is necessary.Based on the daily precipitation data of 44 stations from 1960 to 2007 over Xiangjiang River Basin, the extreme precipitation threshold for every station is defined and temporal,spatial characteristic and inter-annual inhomogeneities of annual extreme heavy precipitation events are analyzed. Firstly, yearly rainfall over Xiangjiang River Basin is simply analyzed. The result shows that yearly precipitation is increased, but rain days are decreased. Secondly, regulations of the extreme precipitation are discussed from multi-aspect, and the rainstorm events are calculated and analyzed further. The results are as follows:1) The extreme precipitation threshold in Xiangjiang River Basin is between 32.6 and 42.6mm/d, which is significantly over the heavy rain standard(25mm/d) made by State Meteorological Administration. All of the northeast southeast and southwest's threshold are over the Basin's average(35.7mm/d),Nanyue station'is up to 42.5mm/d. It means that extreme precipitation threshold has a positive correlation with elevation. In fact, correlation analysis proves that view.2) Extreme precipitation,extreme rain days,intensity of extreme precipitation,extreme precipitation indices and one-day maximum precipitation all tend to increase in recent 48 years over Xiangjiang River Basin. Among all the indexes, extreme precipitation indices'amplification is slight, few stations of that have no growth or negative growth. The other indexes rise markedly. 3) The annual extreme precipitation events occur mostly in Southeast,Southwest mountain and Nanyue station where the daily maximum precipitation increased obviously. The daily maximum precipitation of Yongxing increased significantly, trend coefficient is up to 14.7mm/10a and has passed the significance testing of 99%. The extreme precipitation events are relatively low. Some North stations'daily maximum precipitation such as Zhuzho,Xiangtan station weakened slightly in recent 48 years.4) The overall characteristic of extreme precipitation over Xiangjiang River Basin is that the heavy rain events were more frequent before 1970s, but each grade rain event was in a relatively weak stage during the 1970s and 1980s, all have obviously increased since the 1990s especially the rainstorm and heavy rainstorm events. In fact, not only the rainstorm events increased obviously but also the affected range is extended significantly. Judging from the seasonal distribution, the rainstorm events concentrate during May～August and the maximum value of annual total frequency occurs after 1990s. Among the extreme precipitation events, the rainstorm and persistent rainstorm events over Xiangjiang River Basin consistently occur in the East,South hilly area and most show an increasing trend, the stations with decreasing trend most concentrate in the Middle and Mid-West of Xiangjiang River Basin. In all the stations, Nanyue and Guidong station are where persistent rainstorm occurs mostly.5) From the average spatial distribution, the annual extreme heavy precipitation events concentration degree over Xiangjiang River Basin weakens from North to South, its concentration period shows "low-high-low-high" characteristic. The changing characteristic of the concentration degree and concentration period exist clear difference in sub-regions:Northwest is where the extreme precipitation concentration degree is relatively strong and hard to be abnormal. Southwest is where the concentration period comes earliest and the concentration degree is relatively weak and easily to be abnormal. Southeast Mountain is where the concentration period comes latest and easily become abnormal.6) Both the extreme precipitation events concentration degree and concentration period have undulately risen. Inside, concentration degree occurs mainly between 1970-1979 and 1990～2007; concentration period changes from 129 to 185, which means that the extreme precipitation over Xiangjiang River Basin mainly concentrate in May,June,July.