Drought Risk Analysis In The Zhanghe Upstream Based On Copula Functions

With the warming of climate and the increase of water demand for social and economic development,drought events occur frequently in the Zhanghe upstream.The drought indicators used to explore drought events involve meteorological drought indicators,hydrological drought indicators,agricultural drought indicators,etc.,which mainly analyze individual types of drought situations through a single drought index.However,drought events usually occur simultaneously with multiple drought types.At present,drought research is mostly based on a single drought index,which does not fully reflect the combined effects of meteorological,hydrological and agricultural drought.In addition,the occurrence of drought events presents the coexistence of multiple characteristic variables,and there is mutual influence between variables.Therefore,in order to comprehensively explore the drought risk in the Zhanghe upstream,firstly,based on a single drought index,the Mann-Kendall test method and Hurst index analysis method are used to analyze the drought evolution process and future drought trend in the Zhanghe upstream;then,the Copula functions is used to construct the joint distribution of two characteristic variables of drought duration and drought severity,and the spatial and temporal distribution characteristics of drought in the Zhanghe upstream are comprehensively explored;based on the single drought index,the Ternary Joint Drought Index(TJDI)is constructed by CVine-Copula function to characterize meteorological,hydrological and agricultural drought,and its applicability in drought analysis is compared;finally,the TJDI index is applied to the drought risk analysis in the Zhanghe upstream.The main contents and results are as follows:(1)Through the M-K method and the H index method,the drought risk of the Zhanghe upstream with a single drought index is comprehensively analyzed.The three single drought indexes consistently showed that the winter drought severity in the Zhanghe upstream increase strongly in the future,and the winter drought situation would face severe challenges.(2)The joint distribution of drought duration and drought severity was constructed by Copula function.The Kendall correlation coefficient and Spearman correlation coefficient between the two variables were all above 0.8,indicating that there was a strong positive correlation between drought duration and drought severity.(3)The spatial distribution of univariate drought characteristic return period in the Zhanghe upstream showed a high-risk area of drought.The spatial distribution of the joint drought feature return period in the Zhanghe upstream showed 2 to 3 high-risk drought areas,and contains the areas where the univariate drought features independently showed high drought risk.The joint distribution comprehensively describes the correlation between drought duration and drought severity.(4)Comparied the TJDI index with the three types of single drought index,the results showed that the correlation coefficient between the TJDI index and the three types of single drought index is above 0.5,which is related.The index can fully identify the drought events identified by the meteorological,hydrological and agricultural drought index alone,and the recognition rate is 66 %.The extracted drought duration period integrates the drought duration period of each single drought index,and characterizes the persistence of drought events.The TJDI index comprehensively considers the combined effects of meteorology,hydrology and agricultural drought.(5)The results of drought risk analyzed in the Zhanghe upstream based on TJDI index showed that the severity of drought in spring,autumn and winter in the Zhanghe upstream would show a strong increasing trend in the future.The univariate drought characteristic return period showed a high drought risk area with a spatial distribution of more than 1,indicated that the TJDI index combined various types of drought events.The analysis of drought risk in the Zhanghe upstream based on TJDI index not only comprehensively considers meteorological,hydrological and agricultural drought,but also characterizes the correlation between drought duration and drought severity,which is of great significance to comprehensively describe the risk of drought events.