Study On Reconstructing Long-term Terrestrial Water Storage Anomalies In The Yangtze River Basin And Analysis Of Its Driving Factors

The Yangtze River basin is an important region for national water supply and economic activities in China.The study of its long series terrestrial water storage changes and driving factors can provide crucial insight into regional water resources management,the monitoring and assessment of flood and drought events,sustainable economic and social development.The launch of the Gravity Recovery and Climate Experiment(GRACE)satellite has provided a new tool for the study of surface material transport.However,the GRACE satellite operation ended in October 2017 and its successor mission GRACE Follow-on(GRACE-FO)was launched in May 2018,leaving one year gap between the two generations of GRACE satellite missions.Prior to the launch of the GRACE satellites,traditional methods could not accurately estimate changes in terrestrial water storage in the Yangtze River basin.In this study,we identify the driving factors of terrestrial water storage changes in the Yangtze River basin and construct a new terrestrial water storage anomalies(TWSA)dataset for the past 40 years.By combining the hydrological,meteorological,land surface model and humanistic data from the two generations of GRACE gravity satellite,the influence of different driving factors on the long-term trend of terrestrial water storage in the Yangtze River basin is quantitatively assessed.Meanwhile,the terrestrial water storage are evaluated and rebuilt at the grid scale,which provides the data foundation and theoretical support for water resources-related studies in the Yangtze River basin.The main research contents and results of this thesis are summarized as follows:(1)In response to the problem of intermittent periods between two generations of gravity satellites,this thesis carries out a study of the intermittent period TWSA filling method.The time series decomposition method is used to decompose the GRACE TWSA and soil water storage into trend,interannual plus sub-seasonal and seasonal signals.The relative contribution of soil water storage to TWSA at the grid scale was taken into account,and the proportionality coefficients of the interannual plus subseasonal signals for different soil water storage products and GRACE TWSA were calculated grid by grid,and the intermittent TWSA data were filled based on the proportionality coefficients and GRACE trend and seasonal signals.The accuracy assessment showed that the TWSA built from multi-source soil water storage products was in good agreement with the GRACE TWSA.The proposed method of filling intermittent TWSA data fully integrates the interannual plus subseasonal signal contained in soil water storage and the intrinsic information(trend and seasonal signals)provided by GRACE,which is robust and physically meaningful.(2)To address the difficulty of quantitatively assessing the influence of different drivers and their interactions on the long-term trend change of TWSA,this thesis conducts a study on the driving factors of long-term trend change of TWSA in the Yangtze River basin based on the geographical detector.The influence of different factors on the longterm trend change of TWSA in different sub-basins of the Yangtze River basin and the effect of interaction were quantified by introducing geographic probes.The results show that the main influencing factors of TWSA trend changes in the upper Yangtze River basin are temperature,snow water equivalent,NDVI,precipitation and reservoir storage.Among the interactions of different factors,the influence of snow water equivalent after interaction with temperature is the largest,at 56.4%.Precipitation,temperature and soil water storage are the main driving factors influencing the trend change of TWSA in the middle Yangtze River basin.It is noteworthy that the single factor of nighttime lighting has the least impact on the change of TWSA trend in the middle reaches,but after the interaction with precipitation,the impact factor was raised to 56.3%.Nighttime lighting,as the most direct measure of urbanization process,indicates that the continuous urbanization in some areas of the middle Yangtze River region has caused changes in the long-term TWSA trend.In the downstream basin,the main drivers of TWSA trend changes are precipitation and runoff,but the influence of single factors,such as nighttime lights after interaction with other drivers,is still not negligible.(3)To solve the problem to reconstruct the complete time-scale signal of long-time series TWSA before the launch of gravity satellites,this thesis carries out a study on the construction of the complete signal of long-time series TWSA.Based on the quantitative discrimination of the influencing factors and their interactions of the long-term trend changes of TWSA in different sub-basins of the Yangtze River basin,three machine learning models are used to the long-term trend signal of different sub-basins of the Yangtze River basin from 1982 to 2002.Based on the amplitude changes of the seasonal signal of GRACE TWSA in the Yangtze River basin,the seasonal signal fluctuations in different regions were evaluated and analyzed.The seasonal signal of the Yangtze River basin from 1982 to 2002 was reconstructed using precipitation,normalized vegetation index,soil water storage plus snow water equivalent.Based on the construction results of different temporal components of TWSA,a TWSA dataset was generated for the Yangtze River basin from 1982 to 2021.The reconstructed TWSA dataset was used to detect the mega-flood and drought events that occurred in the Yangtze River basin in 1998 and 2001,and the different stages of flood and drought occurrence were well captured in time and space.The innovations are as follows:(1)By investigating and using the contribution relationship of soil water storage to terrestrial water storage,a method of constructing interannual plus subseasonal signals of terrestrial water storage at the grid scale is reconstructed,which effectively improves the accuracy of land water storage data replenishment during the interruption of two generations of gravity satellites.(2)A geographical detector was introduced to identify the factors influencing longterm changes in terrestrial water storage and the interactions between them.It was found that the trend changes in terrestrial water storage in the Yangtze River basin were influenced by factors such as glacial melt,reservoir storage,precipitation,etc.,and the interaction between nighttime lights and meteorological elements could not be ignored.(3)Based on the identified influencing factors of long-term variation of terrestrial water storage and their interactions,a long time series reconstruction method of terrestrial water storage in the Yangtze River basin was established,and a month-by-month0.25°×0.25° gridded terrestrial water storage dataset was generated for the Yangtze River basin from 1982 to 2021.The method reconstructs the complete signals of trend,interannual,seasonal and subseasonal signals of terrestrial water storage,and the dataset can effectively improve the detection of extreme drought and flood events in the Yangtze River basin.