Detecting Droughts in Southwest China from GPS Vertical Position Displacements

The solid Earth responds elastically to terrestrial water storage(TWS)changes.Here global positioning system(GPS)vertical position data at 31 stations from the crustal movement observation network of China(CMONOC)from August 2010 to December 2016 are used to detect droughts in Southwest China.Monthly GPS vertical position displacements respond negatively to precipitation changes and TWS changes observed by gravity recovery and climate experiments(GRACE)as well as river water level variations.GPS vertical position anomalies(the non-seasonal term)are well correlated negatively(correlations of about-0.70)with the commonly used meteorological composite index(CI)in China and the GRACE drought severity index(GRACE-DSI),but less correlated with the standardized precipitation evapotranspiration index(SPEI).Compared to CI,GPS vertical position anomalies have the advantage of detecting droughts caused by abrupt precipitation deficits in a short time.GRACE-DSI is less accurate in drought monitoring for some periods due to the missing data,while the severity of abrupt precipitation absent in some cases can be overestimated from SPEI with big variability.This study shows the reliability and advantages of GPS data in drought monitoring.

Analysis of terrestrial water storage changes in the Shaan-Gan-Ning Region using GPS and GRACE/GFO

Both the Global Positioning System(GPS)and Gravity Recovery and Climate Experiment(GRACE)/GRACE Follow-On(GFO)provide effective tools to infer surface mass changes.In this paper,we combined GPS,GRACE/GFO spherical harmonic(SH)solutions and GRACE/GFO mascon solutions to analyze the total surface mass changes and terrestrial water storage(TWS)changes in the Shaan-Gan-Ning Region(SGNR)over the period from December 2010 to February 2021.To improve the reliability of GPS inversion results,an improved regularization Laplace matrix and monthly optimal regularization parameter estimation strategy were employed to solve the ill-posed problem.The results show that the improved Laplace matrix can suppress the edge effects better than that of the traditional Laplace matrix,and the corre-lation coefficient and standard deviation(STD)between the original signal and inversion results from the traditional and improved Laplace matrix are 0.84 and 0.88,and 17.49 mm and 15.16 mm,respectively.The spatial distributions of annual amplitudes and time series changes for total surface mass changes derived from GPS agree well with GRACE/GFO SH solutions and mascon solutions,and the correlation coefficients of total surface mass change time series between GPS and GRACE/GFO SH solutions,GPS and GRACE/GFO mascon solutions are 0.80 and 0.77.However,the obvious differences still exist in local regions.In addition,the seasonal characteristics,increasing and decreasing rate of TWS change time series derived from GPS,GRACE/GFO SH and mascon solutions agree well with the Global Land Data Assimilation System(GLDAS)hydrological model in the studied area,and generally consistent with the precipitation data.Meanwhile,TWS changes derived from GPS and GRACE mascon solutions in the SGNR are more reliable than those of GRACE SH solutions over the period from January 2016 to June 2017(the final operation phase of the GRACE mission).