Continuity of terrestrial water storage variability and trends across mainland China monitored by the GRACE and GRACE-Follow on satellites

Terrestrial water storage (TWS) plays an important role in the global water cycle. Measuring dynamic changes in TWS is essential for water resources management, weather-related hazard monitoring and agricultural production. The Gravity Recovery and Climate Experiment (GRACE) and its following mission (GRACE-Follow on) have provided monthly terrestrial water storage anomalies (TWSA) at a quasi-global scale since April 2002. This study bridges the data gap between the two generations of satellites using Random Forest (RF), Support Vector Machine (SVM) and Artificial Neural Network models over mainland China. We systematically examine the spatiotemporal continuity of GRACE and GRACE-Follow on missions based on the spherical harmonics (SH) and mass concentration blocks (mascons) solutions as well as GLDAS-Noah data across mainland China during 2002-2020. Results show that the GRACE-Follow on data of CSR SH/CSR mascons reduces the uncertainty of TWSA in China from 5.26/6.96 to 3.83/4.65 mm/month, respectively. ML-predicted TWSA fits well with in-situ observations during the gap between GRACE and GRACE-Follow on satellites. Modeled TWSA also shows continuity between GRACE and GRACE-Follow on satellites. During the period 2002-2020, reconstructed TWSA significantly decreased from - 0.03 mm/a to - 1.06 mm/a (p < 0.05) with spatial heterogeneity for the whole of China. In many areas of China, TWSA increased rapidly in the range of 5-15 mm/a (p < 0.05), while north and southwest China experienced a decrease between - 5 and - 25 mm/a (p < 0.05). This study provides bridged GRACE data and reveals the variability of TWSA in mainland China, which may contribute to a better understanding of climate change and water resources management.