Study On Dynamic Change Of Vegetation And Its Response To Meteorological Drought In Nenjiang River Basin

Under the background of global climate change,the frequency and intensity of drought continue to increase,especially drought has a far-reaching impact on the growth and development of vegetation.Nenjiang River Basin is an important industrial and agricultural base and one of the concentrated distribution areas of wetlands in Northeast China.In the past decade,the serious salinization of land,the shrinking area of wetlands and the decline of soil and water conservation function have endangered regional ecological security,water security and economic and social sustainable development.Therefore,the study on the dynamic change of vegetation and its response to drought in Nenjiang River basin is of great significance in protecting the regional ecological environment and dealing with extreme climate change.Based on the MODIS NDVI data set from 2000 to 2019 and the air temperature and precipitation data of 32 meteorological stations in Nenjiang River Basin,this study uses standardized precipitation evapotranspiration index(SPEI)as drought assessment index,and uses Theil-Sen Median trend analysis,Mann-Kendall trend test,Hurst index,land use transfer matrix,correlation analysis and lag analysis to explore the temporal and spatial variation characteristics of vegetation NDVI and drought in Nenjiang River basin.Based on the data of land use types,the relationship between the change of land use types and vegetation NDVI in Nenjiang River Basin was discussed,and on this basis,the correlation and lag effect between vegetation dynamic change and meteorological drought at different time scales were analyzed,and combined with the coefficient of variation,the vegetation stability of different land use types and their resistance and resilience to drought were further evaluated.The main conclusions are as follows:(1)During the study period,the average value of vegetation NDVI in Nenjiang River Basin showed an obvious increasing trend,and the increasing rate was different in time scale.The high value area is mainly located in the northwest of the basin,and the low value area is mainly in the southeast of the basin;the average value of NDVI in most areas of the Nenjiang River basin shows a significant upward trend,and the vegetation change mainly shows a trend from improvement to degradation.In the past 20 years,the marsh area in Nenjiang River basin increased the most,which was 15278km~2,and the grassland decreased the most,which was 23232km~2.Among all land use types,only grassland vegetation NDVI showed a downward trend.(2)The SPEI of Nenjiang River basin showed an upward trend in each month during the vegetation growing period,and showed a wetting trend in most areas from May to September,but gradually slowed down in October,and a weak drought trend appeared in some areas.During the study period,the Nenjiang River basin is dominated by global drought,but the influence range of drought shows a narrowing trend.On the whole,the intensity of drought is the highest in the south and southwest of the Nenjiang river basin and smaller in the northwest and southeast.(3)There is a positive correlation between NDVI and SPEI in most areas during the vegetation growth period in Nenjiang River basin,in which the areas with significant positive correlation are mainly distributed in saline-alkali land,grassland,swamp and cultivated land in the southwest of the basin,while the areas with weak positive and negative correlation are mainly distributed in the woodland in the northwest of the basin.The proportion of positive correlation area of NDVI and SPEI is the largest in summer,and the proportion of negative correlation area of NDVI and SPEI is the largest in spring.(4)The areas with significant positive correlation between NDVI and SPEI of corresponding lag time in each month of vegetation growth period in Nenjiang River basin are mainly distributed in grassland,saline land and construction land in southwest and central and eastern parts of Nenjiang River basin,while areas with no significant positive correlation are mainly cultivated land,water area and swamp land,and areas with negative correlation are mainly located in woodland in northwest China.The winter drought has little effect on the vegetation growth,the vegetation growth in the southwest of the basin will be significantly inhibited by the previous year and spring drought,and the spring and summer drought in the southeast of the basin will significantly inhibit the autumn vegetation growth.(5)The stability of woodland vegetation in Nenjiang River basin is the highest,while that of saline-alkali land and water area is weak;under the double disturbance of natural environment and human activities,the resistance of cultivated land and construction land vegetation to drought is stronger,and that of water area vegetation is the weakest;the resilience of swamp land,woodland,cultivated land and construction land vegetation to drought is higher,while that of grassland and saline-alkali land vegetation is weak.