*4.2. Spatio-Temporal Analysis of Flood Inundation*

In order to provide support for disaster-resistant activities in the study area, it is crucial to understand the development process of flood events. Therefore, the particularly severe flood incident occurring from May to September 2020 is selected as a flood monitoring case, and the experimental results retrieved by the DBNN model on CYGNSS data are shown in Figures 9 and 10. Before the onset of the 2020 rainy season in the study area, the regions occupied by water bodies are mainly distributed in coastal areas such as Bangladesh, covering about 6.6% of the total land area. From 20 May 2020, the areas inundated by floods gradually expanded to Nepal and the northeast of India with the arrival of frequent rains. The flooded areas accounted for about 17.9% of the total area on 20 June 2020, and reached 28.7% by 20 July 2020. On 20 August 2020, the areas flooded reached their largest, covering around 34.8% of the total area. After 20 August 2020, the flooded areas gradually reduced because of the decrease in rainfalls. Observing the process of flood changes, we could find the increased area, which refers to the difference between the maximum flood area detected during the rainy season and the area of water bodies before the rainy season, accounting for 28.2% of the total study area, mainly in Nepal and the northeastern states of India. In particular, the proportion of the increased flood area in some states is calculated and summarized in Table 5, where Bihar State experienced the most severe flooding event with an increased area accounting for 89.92% of the total area, while Magway State had the smallest increased area, only covering about 5.50%.

**Table 5.** The proportion of the increased flood area to the state area.


**Figure 9.** The spatial distribution of the study area flood inundation retrieved by DBNN model on CYGNSS. The period spans from May to September 2020.

**Figure 10.** The temporal variation process of the flooded area, spanning from May to September 2020. "CYGNSS-DBNN" indicates flood inundation area retrieved by DBNN method based on CYGNSS data, while "SMAP-Threshold" denotes flood inundation area classified by threshold method based on SMAP data.

Through further observation of Figures 9 and 10, it can be found that the flooded areas are mostly located in Bangladesh, Nepal, and northeast of India, which is mainly caused by three factors; i.e., the large quantities of water vapor carried by the southwest monsoons, as well as the influence of topography and water systems. In terms of topography, the region is located at the southern foothills of the Himalayas and on the windward slope of the southwest monsoons. When the southwest monsoon is blocked by the northern mountains, it is forced to lift up and form topographic rain, leading to high rainfall in the study area. In terms of the water system, rainwater from the northern foothills is usually collected in the northeast of the study area, due to the low topography and dense river networks, making the region more vulnerable to the threat of flooding.
