Multiscale gravity measurements to characterize 2020 flood events and their spatio-temporal evolution in Yangtze river of China

Abstract

In 2020 summer, excessive rainfall pushed rivers and lakes to record high levels over the Yangtze River Basin (YRB), but the five flood peaks were absorbed by the Three Gorges Reservoir (TGR). Mass migration in hydrological processes usually causes the Earth's gravity response, wherefore the 2020 flood events provide a good opportunity to assess the capabilities of gravity measurements in capturing the water mass changes at different scales. In this study, we presents two common gravity measurements (i.e., the satellite mission GRACE-FO and the ground gPhone) used to characterize the evolution of regional floods at a basin- and local- scale, which can be attributed to East Asian rainy season (commonly called “plum rain”) event, i.e., extreme climate-induced effects of the regional water balance, and floodwater absorbed by the reservoir, i.e., the man-made Three Gorges Dam (TGD) intercepting flood peaks. The terrestrial water storage (TWS) estimated by GRACE-FO shows a significant positive anomaly (e.g., ∼370 mm in July and August) in around the TGR. The gPhone measured gravity residual of 10–20 µgal between May and September. The results further suggest that GRACE observation does not have obvious advantages in small-scale TWS monitoring, but gPhone records is difficult to detect the source of the TWS mass change 50 km away. In particular, gPhone recorded the high-frequency gravity disturbance directly related to the high outflow (i.e., >31,000 m3/s) of TGD flood discharge, which is likely used as precursor or co-seismic signal to further study the activity of surface landslide and underground faults induced by the vibration or noise source of the water body. This study shows that the combination of multiscale gravity solutions can potentially detect a wide range of extreme climate events triggered by atmospheric circulation patterns, like El Niño Southern Oscillation and Indian Ocean basin warming accompanied by Sea Surface Temperature (SST) and wind anomalies.