*2.3. Study Area*

South Asia is an ideal area for flood inversion research using GNSS-R technology, due to its frequent large-scale flooding inundation. As shown in Figure 2, the subgraph (a) displays the coverage area of one CYGNSS satellite in a day, and the subgraph (b) represents the intercepted area of South Asia ranging from 4◦N–28◦N and 66◦E–96◦E. The northern part of the region is the Himalayan mountains, with an average elevation of more than 6000 meters; the central part is the Great Plain, with dense river networks and numerous irrigation canals; and the southern part is the Deccan Plateau. The total land area of the study area is approximately 3.24 million square kilometers. In addition, the region is situated in a tropical monsoon climate, with the hot season usually from March to May, the rainy season usually from June to October, and the cool season usually from November to the following February. During the rainy season, the southwest monsoons carry large amounts of water vapor as they pass over the warm ocean, leading to extremely abundant rainfall in South Asia. In 2020, the rainy season in South Asia lasted from June to September, causing the region to experience the most severe flooding of the past decade. Therefore, CYGNSS data ranging from May to October 2020 are selected to conduct the flood inversion, as well as the spatio-temporal dynamic analysis by the proposed DBNN model.

**Figure 2.** (**a**) The coverage area of one CYGNSS satellite in a day. (**b**) The study area in South Asia.
