| With the global warming,the frequency and intensity of extreme climate events have gradually increased,and the impact on vegetation growth is also increasing.The Sichuan-Chongqing area has a wide range,and the topographical differences between the eastern and western regions are large,the water-heat combination of extreme climate events in different terrain areas is obviously different.Analyzing the characteristics of extreme climate events and their impact on NDVI in the Sichuan-Chongqing region and different terrain regions is helpful for a more comprehensive understanding of the characteristics of vegetation NDVI change in this background.Therefore,this paper is based on the daily meteorological data of 47 meteorological stations in the Sichuan-Chongqing region from 1961 to 2018,and 27 extreme climate indices were calculated by the RClimDex model,the characteristics of extreme temperature and precipitation indices in the study area were analyzed.At the same time,combined with the MODIS-NDVI data and meteorological data,the spatial-temporal changes of NDVI values were analyzed,and the effects of extreme climate events on NDVI were discussed.The specific conclusions are as follows:(1)In the inter-annual changes of the 16 extreme temperature indices,the overall warming trend in the Sichuan-Chongqing region was obvious.The changes trend of the number of warm nights(TN90p),the four absolute indices(FD0,ID0,SU25,and TR20),the four extreme indices(TXx,TNx,TXn,and TNn),and the length of the growing season length(GSL)in the relative indices of extreme temperatures were all passed the significance test of P<0.05.All of the indices showed a significant upward trend except for the number of frost days(FD0)and freezing days(ID0)which showed a significant decrease.The number of extreme temperature indicators which reach significance in the northwestern Sichuan,southwestern Sichuan,and eastern Sichuan are 11,11 and 8,respectively.The Sichuan Basin is the region where the extreme temperature indices changes most significantly in the eastern region.(2)Under the background of the slight increase in total precipitation in the study area from 1961 to 2018,the number of continuous wet days(CWD),moderate rain days(R10)and heavy rain days(R20)showed a downward trend,but 5 short-term extreme precipitation indices(RX1day,RX5day,R95p,R99p,and SDII)and the number of storm days(R25)showed an increasing trend.(3)From 2000 to 2018,the average NDVI of Sichuan-Chongqing area was0.53,and the rate of change was 0.037/10a,the slight improvement trend was the main one.Southwest Sichuan has the highest average NDVI,the lowest in Northwest Sichuan,but the eastern region has the highest interannual change rate,which was 0.056/10a.Among the NDVI during the year,except for the NDVI in June showed a slight downward change,the rest of the NDVI were mainly in an upward trend,with the most obvious upward trend in May and November.(4)Among the extreme climate indices,the correlation between NDVI and extreme temperature indices were more obvious than that of precipitation indices.And NDVI has a positive correlation with the temperature warming indices and a negative correlation with the cold indices.At the same time,the relevant of the night indices was higher than the day indices.The relationship characteristics and interannual characteristics between the extreme indexes and the NDVI were consistent.It also showed that the correlation between the NDVI and the temperature indices were higher than the precipitation indices.At the same time,the NDVI of January to April were significantly affected by the extreme climate.(5)In the simulation analysed of the NDVI value,the regression equation R~2value obtained by the Lasso model were higher,and the equation has a higher degree of good fitting.Moreover,the fitted NDVI and the remote sensing image inversion NDVI showed a very significant correlation with the image extraction value,suggested the equation is highly reliable.Among the factors influencing the NDVI value selected by the regression equation,the influence of the temperature indices on the fitting of NDVI values in the Sichuan-Chongqing region and each sub-region is higher than that of the precipitation indices,and most of the temperature warming indices has a positive effect on the fitted NDVI,and the cold indices mostly has an inverse effect,which is in good consistency with the correlation analysis.The spatial-temporal changes characteristics of extreme climate events in Sichuan and Chongqing region were analyzed in this paper,and combined with NDVI data,the relationship between extreme climate events and NDVI was further explored and simulated,which can have a more comprehensive understanding of the changing characteristics of NDVI in the study area under the background of global warming and frequent extreme events,provide decision-making references for strengthening the ecological barrier in the upper reaches of the Yangtze River and ensuring the water environment safety in the Three Gorges Reservoir area,and promote the sustainable development of ecology and economy in Sichuan and Chongqing region. |
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