Study On The Temporal And Spatial Evolution Characteristics Of Drought From A Multiple Drought Types Perspective In The Yellow River Basin

Drought is a recurring extreme climate phenomenon,which has caused different damages to the ecological environment,agricultural production,water resources supply,social and economic development of many countries and regions in the world.The occurrence of drought is relatively complex,involving many aspects such as meteorology,agriculture,hydrology,social economy,etc.,and seriously threatening the living environment,social stability and sustainable development of human beings.As a region most affected by drought in China's major river basins,the Yellow River Basin(YRB)has experienced frequent meteorological,agricultural and hydrological drought disasters,with different occurrence time and mechanism of various drought types.Therefore,the study on the temporal and spatial evolution characteristics of drought from a multiple drought types perspective is helpful to reveal the drought characteristics comprehensively and understand drought phenomena deeply in the YRB,which can provide a scientific reference for guiding the basin to formulate drought prevention measures and rational planning and utilization of water resources.Additionally,the frequent occurrence of drought leads to the reduction of crop yield,the difficulty of water consumption for industry and residents' life,and the degradation of ecological environment.Thus,it is urgent to study the drought evolution characteristics under the changing environment in the YRB.In view of the increasingly serious drought in the YRB,this paper studied the temporal and spatial evolution characteristics of meteorological,agricultural,hydrological and groundwater drought systematically from a multiple drought types perspective based on multi-source data,identified the gridded drought trend characteristics,constructed the drought duration-severity-return period curve,expounded the propagation time of meteorological drought to other drought types,discussed the main drought driving factors,and revealed the influence of teleconnection factors on drought.The main work and conclusions are as follows:(1)The meteorological drought evolution characteristics were studied based on the Standard Precipitation Evapotranspiration Index(SPEI)in the YRB.The results showed that the meteorological droughts were increasing in the YRB during 1961-2015,with an SPEI linear tendency rate of-0.148/10 a.Based on the modified Mann-Kendall(MMK)trend test method,meteorological droughts were increasing in spring,summer and autumn on the seasonal scale.Most of the optimal marginal distribution functions of meteorological drought duration and severity were Generalized Pareto(GP)distributions,and the optimal Copula model was Frank-copula in the YRB.In the whole basin scale,the duration-severity-return period curve of meteorological drought showed that the most serious meteorological drought occurred from May 1997 to August 1998,with drought duration of 16 months,drought severity of 12.44,and drought return period of 115.18 years.(2)The agricultural drought evolution characteristics were studied based on MODIS remote sensing satellite data in the YRB.The results showed that the agricultural droughts were decreasing in the YRB from 2000 to 2015.The linear tendency rates of the Vegetation Condition Index(VCI),Temperature Condition Index(TCI),Vegetation Health Index(VHI),Modified Temperature Vegetation Dryness Index(MTVDI)and Normalized Vegetation Supply Water Index(NVSWI)were 0.2/10 a,0.059/10 a,0.133/10 a,0.005/10 a and 0.137/10 a,with the most obvious trend being seen for the VCI.The alleviation of agricultural drought was the result of ecological protection and construction,and large-area afforestation in recent years.The spatial distribution characteristics of agricultural drought showed that the main agricultural drought type was moderate drought,as monitored by different agricultural drought indices.Based on the MMK trend test method,the agricultural drought was slowing down in each season.Based on the dimensionless Skill Score(SS),the optimal agricultural drought ind ices in spring,summer,autumn and winter were VHI,TCI,MTVDI and VCI,respectively.(3)Based on the surface runoff data,the hydrological drought evolution characteristics were studied in the YRB.The results showed that the hydrological droughts were increasing in the YRB during 1961-2015,especially since the 1990 s,with an Standardized Streamflow Index(SSI)linear tendency rate of-0.021/10 a.The results of the MMK trend test showed that the seasonal and annual hydrological droughts were increasing,with different drought trend characteristics in each subzone.Most of the optimal marginal distribution functions of hydrological drought duration and severity were Loglogistic(Log-L)and Generalized Extreme Value(GEV)distributions,and the optimal Copula model was Frank-copula in the YRB.In the whole basin scale,the duration-severity-return period curve of hydrological drought showed that the most severe hydrological drought occurred from November 1996 to June 1999,with drought duration of 32 months,drought severity of 43.29,and drought return period of 23.26 years.(4)Based on GRAC E gravity satellite data,the groundwater drought evolution characteristics were studied in the YRB.The results showed that the groundwater droughts were increasing in the YRB from 2003 to 2015,with a Groundwater Drought Index(GDI)linear tendency rate of-0.013/10 a.The results of the MMK trend test showed that the monthly and seasonal groundwater droughts were increasing,with different drought trend characteristics in each subzone.Most of the optimal marginal distribution functions of groundwater drought duration and severity were GEV distributions,and the optimal Copula model was Frank-copula in the YRB.In the whole basin scale,the duration-severity-return period curve of groundwater drought showed that the most serious groundwater drought occurred from August 2006 to February 2007,with drought duration of 7 months,drought severity of 7.36,and drought return period of 13.58 years.(5)The propagation time from meteorological drought to agricultural drought was 0-3 months in the YRB.The propagation time from meteorological drought to hydrological drought was shorter in summer(3-4 months)and longer in winter(10-12 months).Additionally,the propagation time from meteorological drought to groundwater drought was also shorter in summer(6-7 months)and longer in winter(13-16 months).There are many reasons for the drought development in the YRB,including climate change and human factors.Furthermore,the relationships between teleconnection factors and drought showed that teleconnection factors had statistically significant relationships with drought in the YRB.Meanwhile,the El N i?o-Southern Oscillation(ENSO)had the strongest impact on drought,whilst the sunspot had the weakest influence on drought.