With increasing anthropogenic activities and the escalating impacts of climate change, flood hazards have become more frequent and severe, causing extensive damage to crops, settlements, and infrastructure. Flood hydrologists have long faced challenges in accurately estimating flood depths-an essential parameter for effective disaster response and risk mitigation. Although optical and microwave satellite imagery have significantly advanced flood mapping by providing near-real-time data on flood extent, duration, and progression, they fall short in delivering precise floodwater depth measurements. The use of virtual altimetry stations-repeated satellite ground tracks intersecting inland water bodies-has effectively complemented in-situ river gauge networks, enhancing flood monitoring and mitigation efforts. However, the nadir altimetry observations measure the water surface elevations directly beneath the satellite (at the nadir point), providing a one-dimensional view of the ocean surface and inland water bodies. The recently launched (December 2022) Surface Water and Ocean Topography (SWOT) altimetry Ka-band Radar Interferometer (KaRIN) ability to measure surface height across a swath (up to 120 km), offering a 2-dimensional view, which is very important for the flood hydrologists.

Figure-1 : SWOT spatial coverage for the selected Ganga stretch

The present study focuses on a segment of the River Ganges flowing through Bihar, Northern India, which experienced significant flooding in August 2024. In this study, we utilize the latest Version C of the “L2_HR_PIXC” data product from ascending pass number 245, right swath tile 199R, and scene 100F to process water level information (Figure 1). SWOT completes 292 orbits over a 21-day repeat cycle, with each orbit consisting of both an ascending (north) and descending (south) pass. In March 2024, the first publicly usable version (Version C) of SWOT (Surface Water and Ocean Topography) data was released to the scientific community and the data can be accessed freely from https://search.earthdata.nasa.gov/. “L2_HR_PIXC,” product is the basic product derived from the Single-Look-Complex images (SLC). Time series data for June 06, July 31, August 20, 29 and September 19, 2024 was processed. The pre-processing of data was carried out to remove the outliers and retain the pixels within the range, followed by applying geometric quality flag to eliminate pixels having geolocation issues. Further transforming of the data from WGS84 ellipsoidal height to orthometric height using the EGM2008 geoid model was done to have comparison with the in-situ gauge water level. For the estimation of the water level along the selected river stretch class (4) “open water” was used for the filtering of water pixels (Figure-2). Water surface elevation data from SWOT was correlated with time series observations derived from Sentinel-3 and in-situ CWC gauge data at Hatidah. The water level data from SWOT shows good agreement with in-situ measurements and exhibits a strong correlation with the Sentinel-3-time series, both demonstrating similar trends. The availability of water level data on same date August 20, 29 and September 19, 2024 from both in-situ CWC gauge site and SWOT passes shows a close match. The SWOT-derived water elevation evaluation showed a root mean square error (RMSE) ranging from 0.17 to 0.95 m

Figure-2: Open water class during July 31 and August 20, 2024

From Figure-3 the water surface elevation derived maps the inundation depth on July 31, 2024 can be seen to be varying between 39m and 42m in the upstream and downstream sections. However, on August 20, 2024 due to increase in the flow of water swelling of the river channel and increase in the depth between 40m and 45m in the upstream and downstream sections is observed. Near the Hatidah gauge site an increase of about 5m is observed between July 31 and August 20, 2024. Time series water level data analysis done from pre-flooding (June 06, 20024) to peak flooding (August 20, 2024) highlights an increase of about 8-10 m in this stretch. The availability of depth component from SWOT would be useful information for flood insurance and flood risk assessment.

The results presented here are preliminary, and further investigations are ongoing to enhance the accuracy and robustness of the analysis. As on today the flood extent can be derived from satellites but depth component is critical for flood hazard mitigation and planning. However, more constellations in future are required to minimize the gap in observations and have frequent updates for operational monitoring.

Figure-3: WSE maps during July 31 and August 20, 2024

References:

JPL. (2024a). SWOT mission overview. Retrieved from https://swot.jpl.nasa 668 .gov/mission/overview/ (Accessed: 10.04.2025)

Dhote, P. R., Thakur, P. K., Gaur, K., Garg, V., Kumar, P., & Singh, R. P. (2025). SWOT Mission with Wide-Swath Altimetry: Observations and Insights into India’s Inland Waterbodies. Journal of the Indian Society of Remote Sensing, 1-10.

Chaudhary, A., Chander, S., Surisetty, A. K., Agrawal, R., Agarwal, N., Gupta, P. K., … & Rao, E. (2025). Evaluation of SWOT Beta Geophysical Products in the Indian Region During CAL-VAL Phase over the Land and Oceans. Journal of the Indian Society of Remote Sensing, 1-14.

Maubant, L., Dodd, L., & Tregoning, P. (2025). Assessing the accuracy of SWOT measurements of water bodies in Australia. Geophysical Research Letters, 52(6), e2024GL114084.