Spatiotemporal Evolution Of Vegetation Cover And Its Response To Multi-time Scale Drought In The Haihe River Basin

Vegetation,as an important part of land cover,plays an important role in regulating climate change and maintaining the energy balance of terrestrial ecosystem.In the context of climate change,the frequent occurrence of drought events has a profound impact on social economy and terrestrial ecosystems.Haihe River Basin is the core region of politics,economy and culture in China.In recent years,the occurrence of drought has caused great losses to the vegetation ecosystem in the basin.Studying the spatiotemporal evolution characteristics of vegetation cover in the Haihe River Basin and its response to multi-time scale drought is of great significance to the ecological environment in the region.Based on the data of temperature,precipitation and GIMMS NDVI 3g(the third generation of Global Inventory Modeling and Mapping Studies Normalized Difference Vegetation Index)data in the Haihe River Basin from 1982 to 2015,this paper uses the Standardized Precipitation Evapotranspiration Index(SPEI),Ensemble Empirical Mode Decomposition(EEMD),Mann-Kendall test,Empirical Orthogonal Function(EOF)analysis and time-lag effect analysis to study the spatiotemporal evolution of vegetation cover and its response to multi-time scale drought in the Haihe River Basin.The main conclusions are as follows:(1)From 1982 to 2015,NDVI in the Haihe River Basin showed an increasing trend in the growing season,spring,summer,autumn and winter,and the increasing rate changed over the years.Spatially,the low value areas of NDVI in each season are in the northwest of the basin,the Bohai Rim region and the the urban centers,while the high value areas are different in different seasons and regions.NDVI showed an increasing trend in most areas of the Haihe River Basin in each season.The areas with continuous improvement or degradation of NDVI account for a small proportion,and the future trend characteristics of most areas are anti-sustainable.(2)From 1982 to 2015,SPEI in the Haihe River Basin showed a decreasing trend in annual scale,spring,summer,but showed an increasing trend in autumn and winter.The frequency of moderate drought is the highest in winter,and that of severe drought is the highest in autumn.Spatially,the annual SPEI decreased in most areas.However,on the seasonal scale,SPEI decreased in most regions in spring and summer,and increasedin most regions in autumn and winter.EOF analysis showed that the spatiotemporal distribution pattern of annual and seasonal scale is mainly the consistent type of the whole basin and the reverse distribution type in different regions.(3)In terms of the correlation between NDVI and drought during the same period,the annual scale NDVI and SPEI showed a significant positive correlation only in Chengde and the Bohai Rim.On the seasonal scale,the correlation coefficients in different seasons are different n different regions.On the monthly scale,the response of NDVI to SPEI in the same month is different.(4)In terms of the lagging of NDVI's response to drought,fewer areas occur the interannual lag effect.Spring,summer,and autumn NDVI are significantly positive correlation with the previous season SPEI in some regions.The lag time of NDVI response to drought in different months is different in different regions.