Research On The Reconstruction And Change Characteristics Of Historical Vegetation And Runoff In The Source Area Of ??the Yangtze River

Due to its unique geographical location,the source of the Yangtze River has been deeply concerned by scientists.However,there were few meteorological and hydrological stations in this area,the measured data is short,the long-term runoff and vegetation cover sequences are few.In order to reveal the long-term hydrological and vegetation variation laws in the source of the Yangtze River,the author analyzed the correlation between the tree ring chronologies,NDVI indexes,runoff sequences and meteorological elements in this area.Therefore,based on the tree-ring data and PDSI gridded datasets,the author reconstructed the NDVI history from July to August and runoff history from June to August,and explored their variation characteristics.The conclusions are listed below:1.Based on the tree-ring samples collected in the source of the Yangtze River,the residual chronology(RES)was established through the steps of surfaced,cross dated,and detrend.The results of climate response analysis indicated that the four tree-ring chronologies had good regional representation,and the variation of tree-ring width was significantly affected by climatic factors,which was mainly related to the temperature and precipitation in May and July.2.Based on the correlation analysis between meteorological elements,PDSI datasets and the runoff series in the source of the Yangtze River,the 706 a runoff variation sequence was reconstructed by PDSI datasets.The independent verification method proved that the reconstruction equation was stable and reliable.Analyzed the reconstructed sequence,13 periods of high flow and 15 periods of low flow occurred.The longest periods of high flow occurred during 1513-1573,the maximum runoff occurred during 1310-1327,The longest periods of low flow occurred during 1389-1414,the minimum runoff occurred during 1687-1699.Compared with the runoff,wetting index and precipitation reconstructed sequences near the study area,the distribution of abundance and dry periods corresponded well.3.Wavelet analysis demonstrated that the reconstructed runoff sequence mainly has significant periodic oscillations of 2-6 years,10-13 years,20-26 years,30-50 years and 50-70 years,which were consistent with the ENSO,NAO,PDO,AMO and East Asian summer monsoon change cycles.The spatial correlation analysis of the runoff series with the global sea surface temperature was carried out to verify the influence of these circulation indices on the runoff variation in the source of the Yangtze River.4.Based on the correlation analysis between the meteorological elements,the NDVI index and tree-ring width chronologies in the source of the Yangtze River,the NDVI sequence from July to August was reconstructed.Analyzed the reconstructed sequence,8 vegetation growth periods and 4 degradation periods occurred.The longest growth periods occurred during 1766-1783 and 1794-1811,the longest degradation periods occurred during 1726-1753.Wavelet analysis demonstrated that the reconstructed NDVI sequence mainly has significant periodic oscillations of 2-3 years,3-8 years,32-64 years,which were consistent with the oscillation periods of ENSO,PDO,AMO.5.The effects of drought and flood on the change of NDVI were analyzed.The results showed that the NDVI index was significantly positively correlated with the PDSI index near the source of the Yangtze River.The correlation between different drought and flood grades was divided,there was significant positive correlation between PDSI and NDVI in extreme drought and normal years,and significant negative correlation in extreme wet years.