Natural-and Human-Induced Impacts On Regional Terrestrial Water Storage Changes From GRACE And Hydro-Meteorological Data

In the early 21st century,the breakthrough of determining the Earth's time variable gravity field from satellite geodesy provides unprecedented opportunities for hydrology.GRACE(Gravity Recovery and Climate Experiment)mission,the first satellite mission which can detect mass changes at the Earth's surface directly,has been widely invoked for a range of studies in global climate change.However,it remains a challenge to assess various GRACE products and obtain more robust estimates of terrestrial water storage(TWS)changes due to the lack of in-situ observations in many regions globally.The topics in promoting GRACE applications to disciplines like hydrology,geodesy,oceanography,climatology,in combination with other data sets,are popular currently,with the accumulation of GRACE data as well as their reliability.This dissertation focuses on GRACE applications to hydrology,including assessing uncertainties in various GRACE products,creating an ensemble TWS model and investigating natural-and human-induced impacts on regional TWS variations over mainland China and some specific regions from GRACE,along with hydrological,meteorological and climatic data.The main contents and results are:First,starting from the relationship between mass and gravity,mass changes at the Earth's surface recovered from spherical harmonic coefficients are systematically introduced,including spatial filters and estimation of scaling factors for signal restoration due to leakage.The generalized Three-Conered Hat(TCH)method is employed to quantify the uncertainties among GRACE products.Then,the weights derived from the uncertainties are utilized to build the ensemble TWS model,which benefits for the usage of time variable gravity fields for end-users.From the perspectives of hydrology,the theory and approaches of GRACE applications to hydrology are elaborated based on the water balance equation and the water conservation equation,including the hydrological characteristics of the inter-basin region and the method to characterize droughts from multi-indices,which are the fundamental theories of GRACE hydrology.The uncertainties of five GRACE products from CSR,GFZ,JPL,HUST and GRGS over mainland China are assessed and the multiscale features of TWS changes are examined.The results show that:?scaling factors derived from GLDAS Noah are better to restore leakage signals in coastal areas,while CLM shows significant ability of restoring hydrological information along the main rivers;?TWS variations over mainland China vary with climate.Among various climate zones,TWS over the tropical monsoon region shows the largest variability,followed by the subtropical monsoon zone.Compared to hydrological models of GLDAS and WGHM,GRACE has stronger capability of capturing the hydrological changes in different climate regions;?the root-mean-square(RMSs)of the averaged uncertainties in CSR,GFZ,JPL,HUST and GRGS over mainland China spatially are 13.2 mm,25.8 mm,31.3 mm,26.0 mm and 58.5 mm,respectively.After removing seasonal and secular signals,the uncertainties decrease in all products;?some regions with significant long-term trends are noted:the TWS in north China,southeastern Qinghai-Tibetan Plateau(QTP)and Tianshan region shows obvious declining trends,while the TWS in central QTP,Sichuan basin,the coastal areas of south China and northeastern China displays increasing trends.In particular,TWS deficits in north China are concentrated in autumn,whereas water increases in the coastal areas of south China are mainly in winter.The teleconnections between TWS changes over mainland China and ENSO/IOD/PDO are investigated based on the cross-correlation analysis.The results indicate that:?TWS changes over the coastal areas of south China and northeastern China are positive correlated with ENSO,while negative correlations between TWS over southwest China,south QTP,Tianshan region and ENSO are found.Generally,the connections between TWS and ENSO are stronger than those of precipitation and ENSO,and in addition,TWS has longer time lags;?TWS variations over the middle Yangtze are significantly affected by IOD,with zero time-lag.TWS variabilities over central QTP,Altai Mountain and north China are influenced by IOD as well,with a time-lag of 2-8 months.Particularly,TWS variations response faster to IOD than ENSO;as a result,TWS anomalies over those areas affected considerably by both of ENSO and IOD are influenced more by IOD during the co-occurrence of these two events;?negative correlations between TWS over south China,southwest QTP and PDO are noted,with a time-lag of 0-3 months,while TWS signals over northeastern China are correlated positively with PDO with zero time-lag.During the co-occurrence of PDO and ENSO events,the mechanism of TWS reacts to ENSO may be changed due to the modulation of PDO.Hydrological variations in the inter-basin region are complex.TWS changes in an inter-basin region and their teleconnections with Indo-Pacific climate anomalies are studied from GRACE,together with local precipitation,hydrological models and in situ river level data,by taking the Sichuan Basin as an example.The results demonstrate that:?TWS changes in the Sichuan Basin are closely linked to local rainfall and water recharge from the adjacent rivers.The Basin TWS shows an increasing rate of?6.0 mm/year largely caused by the increased precipitation.However,severe droughts in 2006 and 2010 have resulted in dramatic withdraws in the Sichuan Basin TWS,and the contributions of the adjacent rivers are different;?during the 2003/01-2012/07 period,IOD behaved stronger negative correlation(r=-0.66)with TWS than ENSO(r=-0.22).However,for the 2012/08-2015/01 period,correlations between TWS and IOD/ENSO became positive and the influence of ENSO overtook IOD,with a correlation of 0.84,which might result from the modulation of PDO.Monitoring and simulating large-scale karst TWS variations remain challenging.This thesis explores the impacts of extreme droughts and water impoundment from Longtan Reservoir(LTR)on karst TWS changes over Southwest China.The results illustrate that:?karst water storage increases with a total volume of 44.55 km3 from 2003 to 2013,and the LTR impoundment contributes about 36.4%.Significant water absents caused by the 2009-2010 and 2011 droughts are observed by GRACE,with volume losses of 208.8 km3 and 287 km3,respectively;?the evolution of LTR impoundment is effectively captured by GRACE,and correlations between reservoir impoundment and karst TWS are stronger than precipitation in autumn and winter,while the summer TWS is influenced more by precipitation;?because of the complexity of karst systems in Southwest China,TCH is an alternative effective method to obtain reliable karst TWS changes;?the meteorological Composite Index(CI)and Palmer Drought Severity Index(PDSI)can underestimate and overestimate the drought severity respectively,while the nonseasonal GRACE-based TWS deficit is a valuable drought indicator in the karst region of Southwest China.In short,the results of this study can not only be helpful for GRACE data processing and the usage of different products for end-users,but also provide more information for hydrological cycles in inter-basin regions as well as offer a new way to study karst hydrology.Consequently,thsese results will contribute to understand the mechanism of TWS responses to climate extremes in our country.