Research On Hydrological Drought In Songhua River Basin Based On Copula Function And Its Response To Meteorological Drought

Hydrological drought is usually manifested as surface runoff and groundwater deficit,which is closely related to hydrological cycle and water balance.The study of hydrological drought in the Songhua River Basin in Northeast China and its response to meteorological drought is great significance to regional drought prevention,water resources management and daily production.In this paper,the Songhua River Basin is used as the research area,and the probability function is used to simulate and optimize the optimal distribution of monthly runoff at each scale of the four hydrological stations in Dalai,Fuyu,Harbin and Jiamusi,and calculate the hydrological drought at 1,3,6,and 12 month scales.Index SRI,and then use the run theory to identify the drought characteristics of drought events,analyze the temporal and spatial distribution characteristics of hydrological drought in the Songhua River Basin;use five Copula function optimization models to construct a multivariate joint distribution,and explore the frequency of hydrological drought in the Songhua River Basin;The two indexes of scale SPI and SPEI characterize meteorological drought,discuss the basic situation of regional meteorological drought,and use the 3-parameter Logarithm nonlinear function to fit the response relationship between aerobic drought and hydrological drought.The main conclusions are as follows:(1)The generalized extreme value distribution and the generalized Pareto distribution can more fit the watershed multi-scale runoff series,and the calculated SRI series show a significant decreasing trend,indicating that the hydrological drought in the Songhua River Basin will continue to increase,Especially the drought of the Songhua River mainstream will be more prominent.The index sequence has undergone abrupt changes in the mid-to-late 1990 s,with 3-5 main cycles,with the strongest cyclical fluctuations around 30 years.The late 1960 s,mid 1970 s to mid 1980 s,early 1990 s and 2000 s were drought periods.The frequency of spring drought and summer drought is relatively high,and the duration,severity and peak value of winter drought are relatively small.The 1-month-scale drought duration in the Songhua River Basin is 5.3-6.8 months,and the severity average is5.0-6.0.The mean value of the drought characteristic sequence increases with the increase of the scale,and the longest drought lasts up to 6 years.Over time,the maximum severity can reach 74.3.Among the 4 basins,the number,duration and severity of droughts in the Nenjiang River Basin and the lower Songhua River mainstream are greater,followed by the second Songhua River Basin,and the middle reaches of the Songhua River mainstream have less drought.(2)There is a strong correlation between drought duration,severity and peak value.Burr,Lognormal,Log-Logistic(3P)and Gamma can fit the drought characteristic sequence well.The applicability of the Copula function in the Songhua River Basin is good.Frank Copula can more fit the two-dimensional joint distribution of drought in the four hydrological stations in the Songhua River Basin,followed by Gaussian Copula and Gumbel Copula.In the three-dimensional joint distribution,t-Copula has the best fit for the duration,severity and peak value of Dalai Station,Fuyu Station,Harbin Station and Jiamusi Station.Analyzing the frequency of hydrological drought in the Songhua River Basin,it is found that the recurrence period of drought events on a one-month scale generally does not exceed 10 years,and the recurrence period of the average drought characteristics generally does not exceed 5 years.Low-peak drought events generally have a recurrence period of no more than 2 years.The main stream of the Songhua River has strong resistance to the "or" and "and" events of drought characteristics.(3)The annual meteorological drought period and hydrological drought period in the Songhua River Basin are basically the same.The summer drought has the highest frequency and strong level.The meteorological drought in the basin showed a slight trend of insignificant relief,and a sudden change occurred around 1990.There were three main periods of time scales of 5a,10 a,and 30 a.There is a strong correlation between meteorological drought and hydrological drought in the Songhua River Basin.The occurrence time of hydrological drought is slightly behind that of meteorological drought.The frequency of meteorological drought is higher than that of hydrological drought,but the mean value of drought characteristics is smaller than that of hydrological drought.Through the establishment of meteorological-hydrological drought duration and severity response models,it can be seen that if only a single precipitation factor is considered,the critical conditions for the transfer of meteorological drought duration to hydrological drought are D=3.14 and S=3.60.After comprehensive consideration of precipitation,evaporation and temperature,etc.,The critical condition becomes D=2.12 and S=4.29.Using SPEI index to characterize meteorological drought is more conducive to the early warning of hydrological drought in the Songhua River Basin.