Construction And Application Of Standard Groundwater Index

Drought is one of the most crucial natural disasters.Traditional drought research focuses more on meteorology drought,agriculture drought and hydrology drought,but less on groundwater drought.With the increase of groundwater usage and problems caused by the decrease of groundwater reserves,the study of groundwater drought,especially in arid and semi-arid areas,has become increasingly important.Therefore,the construction of a groundwater drought index was of great significance to the monitoring and control of groundwater drought.In this paper,the middle reaches of Heihe river basin(MHRB),Shiyang river basin(SRB)and Baojixia irrigation area in the arid and semi-arid region of northwest of China were taken as research areas.Based on the monthly groundwater level sequence of 92 Wells from 1980 to 2018,the standardized groundwater index(SGI)was constructed based on the two methods of parameterization and nonparameterization respectively.The indices obtained by different methods were compared and analyzed.The types of groundwater drought were classified by k-means clustering.The spatial distribution of groundwater drought was studied by empirical orthogonal function(EOF).The periods and trends of different types of groundwater drought were identified based on empirical mode decomposition(EMD).The influence of natural and human activities on groundwater drought was also analyzed.The main results are as follows:(1)Using groundwater level sequence to construct parametric standard groundwater index(SGI).Beta distribution,Gamma distribution,logarithmic normal distribution(Log N),extreme value distribution(EV),generalized extreme value distribution(GEV),Weibull distribution and Rayleigh distribution are selected as the fitting distribution.The K-S test and Akaike information criterion(AIC)are used to optimize the fitting distribution,and it is found that the optimal fitting function in all research areas was Beta distribution.The nonparametric standard groundwater index(NSGI)is also constructed based on the monthly groundwater level sequence,and it is found that the kernel density estimation method can better reflect the variation of groundwater level than the empirical formula method.(2)The parameterized index and the nonparameterized index always show the same trend for one well.It differs only in the smallest detail.Both of them can be used in the analysis of groundwater drought.The advantage of the parametric method is that it has a clear formula and can be extrapolated while the nonparametric method is not.However,for the form of multimodal distribution,the nonparametric method has more advantages.Generally speaking,both methods have their own advantages,which can be used according to the specific situation.(3)The types of groundwater drought based on k-means clustering can be divided into four types: groundwater drought continuously increased severity,groundwater drought didn't occur or drought continued to alleviate,groundwater drought aggravating to alleviating,groundwater drought multiple aggravation and transition of the complex.The groundwater drought is the most serious in SRB.In MHRB,the groundwater drought is lighter than SRB.Baojixia irrigation area has all types of groundwater drought,the groundwater drought is slightest.The spatial distribution of groundwater drought based on empirical orthogonal decomposition is consistent with the K-S results.The areas with continuous aggravation of groundwater drought are often the accumulation areas of Wells with continuous aggravation of groundwater drought,and the spatial and temporal evolution law of drought in other areas can also find the corresponding drought type.Groundwater drought continues to worsen in SRB.In MHRB,the trend of groundwater drought in the south of Zhangye and the south of Linze changed from aggravating to alleviating,but the drought in other areas continued to worsen.Meanwhile,the spatial distribution of the drought in Baojixia irrigation area is relatively complicated.In Liquan,it is obvious that the groundwater drought continues to worsen.In the southern part of Fufeng and the southwest part of Qianxian,the groundwater drought relieves gradually.In other areas,the groundwater drought alternated with its occurrence and disappearance.The overall state of groundwater drought is relatively light in Baojixia irrigation area.Based on the empirical mode decomposition of the standardized groundwater index,it is found that the groundwater drought is dominated by the trend term and less influenced by the period,with a small period of about 1 year and a large period of 3-7 years.(4)Through the correlation analysis of different factors and groundwater drought,it is found that human activities are the main factors affecting groundwater drought,among which economic output,groundwater water consumption and the proportion of groundwater in total water consumption are the three factors with the strongest correlation.Groundwater drought is weakly affected by natural factors,especially precipitation has little correlation with groundwater drought index,while runoff is the factor with the strongest correlation between natural factors and groundwater drought.