Drought-flood Evolution Characteristics And Its Driven Factors Under Water System Connectivity Variation In Three Outlets Area Of South Of Jingjiang River In Yangtze River

In recent years,under the influence of human activities,the river structure has been simplified,which has reduced the flood control and drought resistance functions of the river network to a certain extent.it results in frequent occurrence and an larger influence range of regional hydrological and drought events.In the study,three outlets areas of south of Jingjiang of Yangtze River were taken as the research object,where hydrological drought and flood occur frequently.Under the condition of the change of water system connectivity,the original runoff was standardized and the drought and flood indexes were constructed.The spatial-temporal evolution characteristics and tendency of droughts and floods were analyzed by trend analysis,wavelet analysis,schematic diagram theory,GM(1,1)model and other methods in the base period and change period in the three outlets regions.The correlation relationship between drought and flood index and water system structure,water storage capacity were explored through correlation analysis methods.The main results are as follows:(1)In terms of time,the average annual runoff showed a downward trend in both the base period and the change period,and the decline rates of the runoff during the change period at the four hydrological nodes were 77.2%,55.8%,40.7%,and 65.7%,respectively.The cycle scale wasdecreasing from the base period to the change period and was highly similar in one year period and the wet season.Spatially,the diversion ratio was the same in the base period and the change period,and the order from big to small is Guanjiapu,Xinjiangkou,Mituo Temple,Shadaoguan and Kangjiagang.At the same time,the runoff of five stations was the largest in the rainy season and the smallest in the dry season.In the rainy season,the maximum runoff changed from Kangjiagang in the base period to Xinjiangkou in the changing period.(2)The standardized runoff index(SRI)can visually reflect the changes in hydrological drought and flood characteristics in Three outlets areas of south of Jingjiang river in Yangtze River.The temporal evolution characteristics of droughts and floods was as follows.The drought and flood index of different scales indicated severe drought in 1989,and the trend changes indicated that the probability of flooding in the base period was 48%,mainly in light flood,and the frequency of drought in the changing period was 54%,mainly light drought.Different grades of drought and flood index showed that the frequency curves of drought and flood grades in the base period and the change period were “normal” and“skewed” respectively.The cycle centers of drought and flood index at different times in the change period was smaller,and the period was obviously shortened by 3-4 years.The phenomenon of drought and flood occurred more frequently.In the base period,the duration,intensity andstrength of drought were rising and those of flood were decreasing,while the intensity and strength of drought and flood were rising in the change period.It can be visually reflected that it was drought years in 1964,1978,1994,2006,2012 and flood years in 1965,1998,2008,2016.(3)Spatial evolution characteristics of droughts and floods were as follows.During the base period,the floods in the five hydrological stations were mainly concentrated in the rising water and wet seasons,and were mainly Light waterlogging,moderate waterlogging and heavy waterlogging.Especially the frequency,duration and intensity were maximum value in Xinjiangkou station and the intensity was the larges in Guanjiapu station.During the change period,the frequency of drought in the five stations increased mainly in the dry and falling water seasons.The frequency of special drought and heavy drought increased and extreme drought phenomenon occured in five hydrological station.The frequency of drought in the Mituosi station was relatively large.The frequency of special drought in Kangjiagang station was the largest,and the frequency of drought in Shadaoguan station increased most obviously.In addition,the intensity and strength of the flood were decreasing in the baseline and change periods.(4)According to the GM(1,1)model,the drought years in the next10 years could be 2021,2023,2026,and 2028 and the flood year could be2027.(5)The correlation between drought and flood index in Sankou area and precipitation and evaporation is weak,and the correlation between drought and flood index and runoff influenced by human activities is strong.Among them,the correlation between drought and flood index and water surface rate of water system structure is the largest.The Pearson correlation coefficients were 0.885 in the base period and 0.592 in the change period.There were the largest correlation between the drought and flood index and the tank storage capacity of the water storage capacity,the Pearson correlation was 0.789 in the base period.Correlation relationship between the drought and flood index and the adjustable storage capacity per unit area of the water storage capacity was greatest in change period.The coefficient was 0.591.During the change period,under the influence of water conservancy projects,the structure of the water system changed from complex to simple,the water surface rate decreased and the connectivity among water flows was weakened.These changes resulted in the capacity of the water system reducing in the rising water and wet season.The phenomenon of “small water disaster”occurred easily.In the dry and falling water season,the excess water in the river can not be flowed out to alleviate the drought.