Evolution And Driving Mechanism Of Hydrologic Drought In Wuding River Basin Under Changhing Environment

With the growth of population,rapid economic development and abnormal climate change in the world,frequency and influence scope of hydrological extreme events such as floods and droughts in the Wuding River Basin is on the upward trend.The law of drought changes,and some rivers even break down.Therefore,studying the evolution of hydrological drought and finding out the driving mechanism of hydrological drought in Wuding River Basin under changing environment is significant.This not only can meet the requirements of social and economic production practice,but also has important significance for hydrological drought prediction and planning management.In this paper,the Wuding River Basin is the study area.Based on the hydrological and meteorological sequences from 1956 to 2010,the variation law of precipitation,evaporation and runoff in Wuding River Basin was systematically analyzed.The standardized flow index(SDI)was selected as the index of hydrological drought and the drought grade was divided.Based on this,the evolution law of hydrological drought in Wuding River Basin was analyzed.Based on the driving mechanism of runoff variation,the contribution of climate change and human activities to hydrological drought is studied.The main conclusions are as follows.(1)Characteristics of hydrological elementsPrecipitation shows a non-significant decreasing trend as a whole and a smaller degree of reduction.By means of the ordered cluster method and the Lee-Heghinian jump diagnostic method,the precipitation was mutated in 1964,but the mutation point was not significant by the run length theory,indicating that there was no abrupt change in precipitation.Through periodic analysis,it is found that the evaporation sequence of the non definite River Basin mainly exists in the periods of 3~8,7~15,13~20,15~20 and 23~28,and the main cycles are 3 and 16 years.Evaporationshowed a non-significant decreasing trend and a smaller degree.By means of the ordered cluster method and the Lee-Heghinian jump diagnostic method,the precipitation was mutated in 2001.After the test of run length theory,the mutation point was significant,indicating that the evaporation occurred in 2001.Through periodic analysis,it is found that the evaporation sequence of the non definite River Basin mainly exists in the periods of 5~7,8~15,8~13,15~30 and 25~30,and the main cycles are 10 years and 27 years.The depth of runoff showed a decreasing trend and a smaller extent,but it was tested by the significance of M-K.Through the orderly clustering method and Lee-Heghinian jump diagnosis method,it is concluded that the runoff depth is inconsistent at each site,but all of them are concentrated in 1970 and 1971.By the run-length test,the two mutations are significant Runoff depth occurred in 1970 ~ 1971 mutation.Through periodic analysis,it is found that the sequence of runoff depth mainly exists in 3~7,7~15,10~20,15~20 and 15~25 years,and the main cycles are 10,18 and 20 years.(2)Characteristics of Hydrological Drought EvolutionFrom the analysis of drought duration and drought intensity,some obvious changes have taken place in 1971.Before 1971,the drought lasted mainly concentrated in 0~7 months,the drought intensity is less than 2;after 1971,drought duration has extended trend,concentrated in the 8~12 month,the maximum 36 consecutive months reached mild drought occurred in 2003~2005 years,the drought intensity is aggravated,the most severe drought intensity reached 17.8,in 2006.From the drought time series analysis,the runoff of the river basin in general shows a downward trend.Before 1971,the amplitude of runoff was large.After 1971,the amplitude became smaller,and the vibration cycle was longer,shorter and longer.From drought frequency analysis,the frequencies of different drought grades are slightly different at different time scales.Overall,during the 1956-2010 years,the frequency of heavy drought and drought in the less determined River Basin was low,and the cumulative frequency of drought and light drought on each site could reach 80~90%.(3)Analysis of driving mechanism of hydrologic droughtThe precipitation was consistent with the overall trend of regional hydrological drought index,and had a positive correlation,and the correlation was weak.In the Wuding River Basin,precipitation is the main source of the runoff of the recharge channel.When the extreme weather is encountered,the surface runoff is reduced dueto the serious precipitation,and the river flow or the groundwater level is reduced,leading to the occurrence of hydrological drought.There is a negative correlation between evaporation and regional hydrological drought index,that is,when the amount of evaporation is greater,the hydrological drought index is smaller,and the correlation coefficient is small,not significant.In the Wuding River Basin,river runoff evaporation is a large expenditure,when the rainfall is average,evaporation increased,the surface runoff decreased,underground water level decrease,resulting in hydrologic drought.From the driving mechanism,the impact of human activities is 81.6%,while the impact of climate change is only 18.4%.Therefore,human activities are the main drivers of runoff reduction.And because the runoff is a reflection factor of hydrological drought,the change of runoff can reflect the drought in a certain sense,so the human activities in the Wuding River Basin have the influence of hydrological drought.