Flood Inundation Extraction and its Impact on Ground Subsidence Using Sentinel-1 Data: A Case Study of the "7.20" Rainstorm Event in Henan Province, China

On July 20, 2021, the northern Henan Province was hit by a historically rare, exceptionally heavy rainstorm ("7.20" Rainstorm Event), accompanied by severe urban flooding, flash floods, landslides, and other multiple disasters, resulting in significant casualties and property losses. On the other hand, the long-term overexploitation of groundwater since the last century has led to severe ground subsidence in the same area. We apply the intensity information of Sentinel-1 SAR images to extract the large-scale flood inundation area and their phase information to measure the ground deformation. Since heavy precipitations can recharge groundwater, the relationship between flood inundation, groundwater level change, and ground deformation is analyzed. The results show that the flood inundation areas are mainly distributed along the major rivers due to river overflowing, while heavy precipitation led to the rise of groundwater levels, and there was a significant amount of subsidence mitigation and surface uplift across the region due to the groundwater recovery. This study demonstrates the contribution of radar remote sensing to analyze the mechanism of groundwater recharge and subsidence mitigation benefited by rainstorm events and provides a technical reference to similar circumstances.