Seasonal Variation Of Terrestrial Water Storage In Yunnan Inferred From GPS Observations Of Vertical Land Motion

With the development of science and technology,the space geodetic measurements present diversity and high precision,and Global Positioning System(GPS)has been applied in various fields for its high precision,real-time and all-weather characteristics.Several researches used GPS vertical displacements to obtain terrestrial water storage variation in America,and results suggested that GPS show good performance,especially in matter of spatial resolution with respect to GRACE,hinting that GPS data could provide a new technical means to monitoring of terrestrial water storage.The construction of GPS continuous observation network in our country is very perfect,and it is important to explore the feasibility of applying current GPS network to monitor the change of land water storage in China.Most recent researches show that obvious seasonal crustal vertical deformation recorded by GPS is largely related to mass redistribution on the Earth's surface.With the GPS vertical observations at 47 stations in the Crustal Movement Observation Network of China(CMONOC),we investigated the land water storage change in Yunnan Province during 2010-2015,As a conventional trade-off procedure,the optimal smoothing parameter is obtained by using L-curve method to balance the observation misfits and solution smoothing functions.The inversion results of Yunnan province terrestrial water storage obtained from GPS data showed the obvious geographical characteristics:the annual variation of water storage in the southwest of Yunnan is higher than that in the eastern plain,showing less variations from west to East and from low latitude to high latitude;In the time scale,the results showed that most areas of Yunnan Province,seasonal changes of water storage in October(about the end of the summer)reached the maximum,while present minimum value in April(end of winter);the water storage changes in Yunnan Province showed a slow increasing trend,at a rate of about 20mm/yr,based on the inferred results during 2010-2015.In addition,the GRACE,GLDAS and TRMM data are collected to compare with the GPS-inferred total water storage,and the results show that the time series of the change of terrestrial water storage in Yunnan province obtained by GPS inversion has a high consistency with GRACE and GLDAS data in the spatial and temporal distribution(the correlation coefficients of GPS and GRACE and GLDAS were 0.80 and 0.76,respectively).It is evident that the GPS data could be used to retrieve the change of the terrestrial water storage as an independent geodetic technique,and,moreover it has the advantage as follow:(1)relative to GRACE 300km spatial resolution,dense GPS network can improve the resolution of the data grid;(2)GPS data is nearly close to real time,the data delay is only one day,while the GRACE data delay for about a month,which can improve real-time monitoring;(3)GRACE due to its operation and maintenance needs lead to data loss(lack of data during the period of December 2010-2015)which can be recovery by the scale factor from GPS and interpolation and the change trend information provided by GPS;(4)the terrestrial water storage obtained by GPS inversion include the groundwater and lake water in GLDAS.However,the inversion results depend greatly on the density of the GPS stations and the accuracy of the GPS observations.At present,the domestic GPS observation network is not perfect;what's more,systematic signals are still evident after removing the environmental mass loading.The systematic residuals are probably attributed to GPS related errors,such as the draconitic errors which is related to GPS draconitic year with a period of 351.2 days.Further analysis is need to explore and carried out to valid the ability of GPS data to infer the terrestrial water storage before using it to hydrological application.