Variation Characteristics of Extreme Low Runoff in Midstream Of Ganjiang River and Its Influence Factors

The impact of climate change on extreme hydrological events is a matter of worldwide concern. Ganjiang River is the biggest river in Poyang Lake Basin, and also an important tributary in the lower reaches of the Yangtze River. In recent years, water shortages often occur in the middle and lower reaches of Ganjiang River during the dry season, which the region's industrial and agricultural production and people's life were greatly influenced by. Therefore, under the background of climate change, the change trend of runoff in the middle and lower reaches of Ganjiang river, especially the evolution of hydrological extreme events, has an important guiding significance for drought mitigation. In the paper, based on daily runoff data in the Xiajiang hydrological station from 1957 to 2012 and daily precipitation data in 6 meteorological stations from 1957 to 2012 in Ganjiang River basin, the characteristics of extreme low runoff were analyzed by using the non-parametric Mann-Kendall test and Morlet wavelet analysis, and the causes underlying extreme low runoff changes were discussed. The results showed that (1)the extreme low runoff generally experienced four phases during 1957-2012: decreasing - fluctuation - increasing - fluctuation; the extreme low runoff was decreasing before 1969, and then to be a fluctuate change during 1969-1992, the increasing trend was clear from 1993-2002, however, after 2002, the extreme low runoff was a fluctuate change again. (2)the mean and variation coefficient of the extreme flow varied greatly at different statistical period; after 1970s, the inter-annual variation of the extreme flow was larger. (3) the minimum 1-.3-,7-day runoff occurred mainly in 12, 1 and 2, and the minimum 30-day runoff occurred mainly in 11, 12and 1, while the minimum 90-day runoff occurred mainly in 10, 11and 12.(4) the extreme low runoff had significantly increasing trends during 1957-2012(p<0.01) with the average rate of　40.26 m³/s·10a ^–1.(5) the abrupt change points for extreme low runoff occurred in 1969; compared to 1957-1969，mean value and variation coefficient of the extreme low runoff increased during 1970-2011 with the range of 60.26-70.18% and 23.49-71.74%, respectively.(6) the wavelet analysis discovered that there existed 4 periodic oscillations: short oscillation period of 2~3, 5-8, middle oscillation period of 9~14 and long oscillation period of 23-25, but all were not significant.(7) the extreme low runoff was controlled by annual precipitation, summer precipitation and autumn precipitation; there was a significant positive correlation between the precipitation and extreme low runoff, which means that precipitation was the main reason for change of extreme low runoff in downstream Ganjiang River, but vegetation coverage changes and construction of water conservancy project also plays an important role in the regulation of the low flow.