"Specular and Quasi-Specular Echo Analysis for Flood Monitoring in Pakistan's Indus River Basin"

Continuous monitoring of water extent at inland water surfaces is necessary during any season. However, it is not often possible due to the unavailability of flood gauges, inaccessible locations, and other hazardous situations in flooded areas. Despite this, advanced satellite radar altimetry-derived water surface elevations may be utilized to supplement in-situ data. Synthetic Aperture Radar (SAR) high-resolution mode has replaced pulse-limited low-resolution mode (LRM) in satellite radar altimetry in recent years. In Pakistan, radar altimetry satellite data is used to find and compare the level of the water surfaces of the River Indus near the Sukkur barrages and analyzed that altimetry performed better in flat areas as compared to mountain areas. In this study Sentinel 3 L2 datasets on inland water surfaces were evaluated and applying pulse masking using the global water surface explorer facilitated isolating the pulses in the waveform over water and land in Sindh near Sukkur Barrage for a rainy year August 25, 2022 (Inundation Period). Echoes were found highly specular during inundation while quasi-specular during dry months. Waveform shapes were separated in different months. Time series of the water level of these months were also evaluated and compared with In-Situ gauge data taken from the Sindh Irrigation Department (SID). The accuracy evaluation uses variate differences, pass-to-pass repeatability, and comparison to gauge measurements. We have proposed a straightforward rule to separate Specular, Quasi-Specular, and non-Specular echoes for floodplains. Furthermore, radar altimetry may provide information about flood water depth. This study will help to map inundation extent and depth at inaccessible locations during all weather conditions.