Vertical Loading Displacement Of The North China Plain Inferred From Satellite Gravimetry

The water on earth is interrelated and exists in three forms:liquid,solid,and vapor.The seasonal migration and redistribution of water will cause the vertical loading displacement to change to varying degrees,correspondingly,the earth gravity field will change.Since GRACE gravity satellite launched in 2002,the resolution and accuracy of the earth's gravity field model which determine by the corresponding satellite tracking satellite data had been further improved.Especially the time-varying gravity field model determined by GRACE played an important role in monitoring changes of terrestrial water reserves,the quality of ice,sea level,ocean circulation,disasters,etc,.Over the past 50 years,groundwater in north China plain has been over-exploited seriously,which causing a large area of ground-subsidence in this area.In this paper,I calculated the change of vertical loading displacement in north China plain,and explored the periodical characteristics and its long-term trend,which based on the model of GRACE time-varying gravity field.At the same time,I estimated the change value of vertical loading displacement in north China plain by using the GLDAS hydrological model.The results of both calculation showed that there was obvious periodical change characteristics,and the long-term change trend was also similar.There is good correlation and its Pearson correlation coefficient is 0.62.The annual amplitude and anniversary phase have similarity in spatial distribution.The annual amplitude is shown to be reduced from high latitudes to low latitudes,while the anniversary initial phase estimated by GLDAS data is one month later than results calculated by GRACE.The vertical loading displacement of the entire north China plain changed significantly in 2009,2011 and 2013.The average cumulative change in 2009 was 7.5mm.In 2011,the cumulative change of vertical loading displacement in the high-latitude areas which is bounded by Shijiazhuang and Dezhou reached-8mm.In 2013,the annual cumulative change in the low-latitude areas bounded by xingtai and jinan reached 8mm,and the cumulative change of vertical loading displacement in the high latitudes bounded by shijiazhuang and tangshan reached-4mm.During the rest of the study period,the total change value of vertical loading displacement in the whole north China plain was small.In terms of the rate of change,the change rate of vertical loading displacement between January 2008 and August 2009 was +0.49mm/a.As the total amount of rainfall increased from September 2009 to August 2011,the vertical loading displacement of the earth's surface decreased in this period of time,and its annual change rate was-0.79mm.The annual change rate of vertical loading displacement between September 2011 and June 2012 was +2mm,while it was +lmm between August 2012 and December 2013.At the same time,I compared the direct observation results of 23 GPS network engineering reference stations in China with the vertical loading displacement change value calculated by GRACE data.The results show that most of the stations show a consistent trend between them.The percentage of stations which correlation coefficient is over 0.3 is 53%.About 30 percentage of stations' correlation coefficient is between 0 and 0.3.In addition,there are 17%of stations which correlation coefficient is negative.The average RMSE in the southern region was 4.72mm,while it was 6.22mm in the northern region.It is shown that in the southern region where rainfall is abundant,the calculation results of GRACE data is better than that in the relatively dry northern regions.As a result,it is further explained that GRACE can monitor the change of vertical loading displacement in large scale river basin as a single observation measure.