Evaluation of water storage variability and water balance in the karst areas of Southwest China using GRACE data

Abstract

Karst is an important landscape on the earth. The southwest China encompassing Guizhou, Guangxi, and Yunnan provinces is a typical karst region where nearly 40% is covered by karst landscape. This thesis aims to explore the terrestrial water storage (TWS) variability and water balance in southwest China using state-of-the-art satellite gravimetry. The data used mainly include (1) the spherical harmonic coefficient and mascon solutions from the Gravity Recovery And Climate Experiment (GRACE) and its succeeding Follow-On mission, (2) the simulated water storage anomaly from hydrology and land surface models, (3) the reanalysis-based TWS anomaly (TWSA), total column water vapor and vertical integral of the divergence of moisture flux, (4) satellite-based ET, and (5) ground-based precipitation, streamflow, soil moisture, and groundwater levels.

The objectives were to address the following issues. Firstly, the sensitivity of leakage correction of GRACE data using a priori model simulations is analyzed. An inter-comparison is performed for leakage-corrected TWSA using additive, scaling factor, and multiplicative methods, the mascon-based and in situ-based TWSA. Secondly, the GRACE data and in situ groundwater level data are used to investigate the groundwater storage (GWS) dynamics in the highly karstic (Guangxi and Guizhou, with ~50% of karst) and low karstic (Yunnan, with 28% of karst) regions. Thirdly, the groundwater recharge and baseflow in the Wujiang and Xijiang River basins are estimated using GRACE and in situ data. The groundwater recharge and discharge mechanisms at the monthly, seasonal and interannual scales are discussed including during dry periods. Fourthly, GRACE-based TWS change data are used to quantify the interbasin groundwater flow in the highly karstic Wujiang River Basin (>70% of karst) using the terrestrial and land-atmosphere water balance approaches. Fifthly, the long-term TWSA from GRACE and the Follow-On missions, and the Modern-Era Retrospective Analysis for Research and Applications (version 2) in the Pearl River Basin is examined and then used to characterize the drought and flood using water storage deficit index and flood potential index. Further, four climate indices (i.e. El Niño-Southern Oscillation, North Atlantic Oscillation, Indian Ocean Dipole, and Pacific Decadal Oscillation) are used to explore their teleconnection with TWSA and precipitation using the cross-correlation method.

The key findings of this thesis are as follows. Firstly, the near-equal performance of mascon and spherical harmonic solutions is further corroborated. Secondly, TWSA, recharge and discharge under different karst hydrogeology conditions is distinguishable, especially during drought periods. Thirdly, a water budget closure is achieved when considering the interbasin groundwater flow in Wujiang River Basin. Fourthly, GWSA and recharge estimated from GRACE data are different from those from in situ data owing to the different hydrogeology environments at different spatial scales. Fifthly, the historical reanalysis-based TWSA data in the Pearl River Basin before 2000 are unreliable. The drought severity in the Pearl River Basin has intensified over the last four decades. GRACE captures the spatial pattern of the 2008 flood in the Pearl River Basin. El Niño-Southern Oscillation is the major climate index that influences the droughts and floods in the Pearl River Basin.