Investigations On Time-variations Of Continental Water Storage With GRACE Gravity Models

The earth's gravity field reflects the spatial distribution, movement and change of the earth's mass. Research and determination in the fine structure of the earth's gravity filed and its time-variation is one of the main tasks of physical geodesy. And, the central work in determination of gravity field is to build global gravity model and global geoid, especially to establish regional geoid model with high precision and high resolution. Along with the abundance of gravity data in the earth surface, satellite altimetry data,satellite gravity data ( SST, SGG), digital terrestrial model (DTM) and other related information of the earth's gravity field, the determination of the earth's gravity field will develop towards time-variation, high precision and high resolution from static, low precison and low resolution relatively. Besides, the research in gravity field will concentrate on applications. For example, the investigation on global ocean circulation, the study in polar ice mass balance, and so on.The earth is a complex dynamic system. The redistribution of the earth's mass will lead to the change of gravity field in different time scale. Gravity observations with high precision and resolution may provide valuable information about the movement and exchange of the earth's mass. In the investigation of substance movement, the variation of continental water storage plays an important role in the research of global climate change. The interrelated information got from many techniques play great economic and social values for the application of water resources including evaluation and improvement of climate model, the understand of hydrology process in large scale, and the monitor of water use in agriculture. The development of spacial techniques, especially the implement of satellite gravity mission, provides the possibility to detect global mass movement and exchange.In this paper, based on the Level-2 data product provided by GRACE satellite gravity plan, the change of continental water storage in China is analyzed in. And, the key issues in the selections of Gaussian smoothing radius and truncated degree of gravity model are discussed. Then, combined with Principal component analysis (PCA) method, J₂ items of GRACE Level-2 and SLR are studied, whose period characters are obtained and then compared. Finally, the period characters of continental water storages in Amason basin and Yangtse basin are investigated with PCA. Besides, the annual amplitudes are also computed.In the introduction, the development of satellite gravity is introduced briefly.In chapter 2, firstly, the scientific objectives, the payloads and the data products of GRACE mission are described. Secondly, the dynamic method in computing the gravity field model with GRACE data is expatiated. The development of GRACE gravity field model is also introduced. Besides, the GRACE data product adopted in the paper is explained. Thirdly, GRACE gravity field models (GGM02) are compared to other gravity models.In chapter 3, the methods of deriving the mass variation of the earth from GRACE time-variable gravity model are introduced. And, the key issues including Gaussian filter, the choice of filter radius and truncated degree as well as the influence of J₂ are discussed. Then, based on the validation of adopted method by comparing to the hydrology model, the variations of water storage in China are inferred, and the results are analyzed.In chapter 4, the fundamental of PCA is expatiated. Then, the time series of J₂ item got from GRACE and SLR are analyzed respectively with PCA, and the results are compared in order to seek the data processing method of J₂ item in the process of inferring regional continental water storage variation. Lastly, the variations of water storage in Amason basin and Yangtse basin are analyzed with PCA, and the main time-variable characteristics and annual amplitudes are acquired.The conclusions and outlooks are given in chapter 5.The main research achievements are as follows:1. Precision analysis of GRACE gravity field model. The analysis shows that, improvements relative to past gravity model are clearly achieved below 90 degree for GRACE gravity model. And, to a certain extent, GRACE satellite gravity plan has settled the gravity measurement in continental gaps effectively as well as in polar region.2. The key issues in inferring the mass variation of the earth including the choice of Gaussian filter radius and truncated degree as well as the influence of J₂ are investigated in detail. The results show that for Gaussian smoothing radius of 800km and truncated till 20, the result is good relatively, and the influence of J₂ item of GRACE gravity model is important.3. The variation of global water storage is inferred. Compared to CPC hydrology model, accordance is achieved in Amason basin, Niger Lake, Chada and Zambia basin, Bengal bay and Yangtse basin as well as the north part of Australia.4. The variation of continental water storage in China is studied. The variation of continental water storage in China shows clear regional imbalance and seasonal change.5. The time series of J₂ got from GRACE and SLR are analyzed with PCA respectively. The analysis indicates that J₂ item of SLR has better seasonal variable characteristics than that of GRACE.6. The variation of continental water storage in Amason basin and Yangtse basin are analyzed with PCA. The analysis shows that, annual amplitudes of the variation of continental water storage reconstructed with PCA in Amason basin and Yangtse basin are 20.1 cm and 4.9cm respectively.