The Multi-spatiotemporal Scales Climate And Runoff Reconstruction Of The Upper Yangtze River Based On Tree-ring

The Yangtze River’s upper reaches are the primary channel of the river above the Yichang section.The Yangtze River’s main stem and other tributaries,including the Yalong River,Min River,and Jialing River,make up the upper parts of the Yangtze River system.And the catchment area of the Yangtze River system is primarily located in the center of the Qinghai-Tibet Plateau-the eastern edge of the area（the western Sichuan Plateau）,which is one of the largest and highest altitudes natural wetlands and biodiversity distribution areas in China,and is also China’s main water source and the country’s ecological center.This area is also susceptible to climate change,which has a substantial effect on downstream in addition to having an impact on the region’s biological environment.As a result,studying climate and hydrological changes in the Yangtze River’s upper reaches is critical.However,the series of climate and runoff variations observed are short and therefore revealed trends have limitations regarding their explanatory power,leading to an incomplete understanding of the hydrology and climate.In this paper,we first reconstructed some regional temperature in the study area,and then,the fieled temperature was reconstructed and analysised in the upper Yangtze River and surrounding areas over the past several hundred years based on tree-ring chronologies;finally,reconstructions and analyses of the Yangtze River source and upper Yangtze River runoff were performed using multi-source indicators.The following are the paper’s main conclusions.（1）Using tree ring data from spruce sample sites in the vicinity of the upper Yalong River,the history of summer maximum temperature in the middle Yalong River from 1541 to 2019 was reconstructed.There were 34 extremely warm years（7.2%of total years）and 36 extremely cold years（7.5%of total years）during the reconstruction period.The results of the ensemble empirical mode decomposition（EEMD）analysis indicated quasi-oscillations of 2.9-4.2 years,4.5-8.3 years,11.1-15.4 years,20-33.3 years,50.4 years,159.7 years,and 250 years in this temperature reconstruction which may be associated with El Ni（?）o Southern Oscillation（ENSO）,solar activity,and Pacific decadal oscillation（PDO）.（2）Based on the ring width data of Abies and larch sampled in the lover Sea Scenic Area of the Dadu River,we established the standard chronology of the two tree species,and discussed the response relationship between the radial growth of the two tree species and meteorological factors and the time stability of the response relationship.The correlation analysis between chronology and climatic factors showed that there were differences in the response of radial growth of Abies and larch to climatic factors.The precipitation in previous December significantly inhibited the radial growth of Abies,and the precipitation in current July significantly promoted the radial growth of Abies;Precipitation has no significant effect on the radial growth of larch.The radial growth of Abies is greatly affected by the lowest temperature in current June and July,showing a significant positive correlation;Larch has a significant positive correlation with the maximum temperature and average temperature in current May.The sliding correlation shows that the significant relationship between the radial growth of Abies and the precipitation in December and July of the previous year and the temperature in June of the current year has remained stable;the relationship between larch radial growth and climate conditions,in contrast,was unstable.Based on this,the upper Dadu River region’s summer minimum temperature variation was recreated using the Abies width chronology.（3）We present a tree-ring network including 48 chronologies(oneδ¹³C chronology,one regional maximum latewood density chronology,and 46 width chronologies).Using the modified point-by-point reconstruction method,a temperature field of 21 meteorological stations in the upper Yangtze river region over the past 200–500 y（the common period was from 1730 to 1998;the longest single period was from 1480 to 2002）was reconstructed.Principal component analysis and rotated principal component analysis results indicated that the reconstructed temperature field may be representative of spatial distribution characteristics.Temperature changes were mainly synchronous with those in the middle and low latitudes of the Northern Hemisphere.We then explored the influence of possible climatic forcing and found that the Indian Ocean Basin mode may be a principal factor controlling the interannual variations in the summer temperatures.（4）Tree-ring width data collected at five sites near the source of the Yangtze River were used to construct a regional chronology.A record of the annual runoff of the Yangtze River at its source from 1374 to 2012 was then reconstructed.The reconstruction indicated that 17 periods of high flow and 14 periods of low flow have occurred over the past 639 yr.The wet and dry periods in the reconstruction coincided with similar periods identified in a time series of Palmer drought severity index values obtained near the research area,and the reconstructed streamflow in the upper Yellow River.Several important periods of low flow also coincided with periods of drought that affected broader regions.An analysis using the multi-taper method demonstrated that periodic variations in runoff of 2-5 yr,ca.20 yr,and ca.50 yr have occurred at the source of the Yangtze River.Furthermore,solar activity and the of the South Asian summer monsoon intensity are important factors influencing runoff variability.（5）Using a combination of tree-ring width chronology of the Yangtze River source,oxygen isotope chronology of the middle part of the upper Yangtze River and the summer precipitation grid data set of the past 544 years in Asia,a random forest method was used to reconstruct the 1470-2013 flood period runoff（June-September）at the Yichang hydrological station.The reconstructed model was able to explain64.6%of the variance of runoff during the calibration period,and the reconstructed runoff series were validated by the reconstructed Brahmaputra River runoff,PDSI reconstruction and paleoflood records.During the past 544 years,the upper Yangtze River region has experienced 11 wet periods and 10 drought periods,among which there were continous periods from the late 16th century to the early 18th century,which can further confirm that there were severe and lasting droughts not only in the north of China during the late Ming Dynasty,but also in the upper Yangtze River region.The results of multi-taper method and wavelet analysis show that there were2-8 years,quasi-20 years,quasi-40 years,70-90 years,and 150-180 years cycles,and ENSO,South Asian monsoon,solar activity,and Atlantic Multidecadal Oscillation（AMO）may be the main circulation factors affecting the runoff in the upper Yangtze River.The reconstructed runoff in the mid-18th century shows a century-scale change,and ENSO variability may be the main control factor for this change.