Monitoring the water level of coastal wetland in Beibu Gulf using Jason-3/Sentinel-3A altimetry satellites

The water level of coastal wetlands is essential to the growth of mangroves in the development of the local economy. The current monitoring system still relies on hydrological stations to obtain the regular water level at fixed positions. However, the high costs of the conventional method cannot meet the harsh requirements in time. Satellite altimetry technology can be widely expected to serve as an effective supplement to ground monitoring. Taking the coastal wetland of Beibu Gulf in Guangxi as the research object, this study aims to propose a feasible monitoring technology for the water level using the daily Jason-3 and Sentinel-3A altimetry data from 2016 to 2020. The initial value of water level was calculated in coastal wetland through the radar altimetry toolbox, where the triple middle error was used to remove the outliers. The obtained initial points of the water level in the coastal wetland were first imported into the Omap, thereby removing the redundancy data of water level falling on the land and vegetation coverage. As such, high accuracy was achieved to prevent the waveforms near shore from the land. Four re-tracking were then used to correct the water level in the height measurement, including the shift of gravity center, threshold value, the improved shift of gravity center, and the improved threshold value. The determination coefficient, the Root Mean Square Error (RMSE), and the Mean Absolute Error (MAE) were selected to quantitatively evaluate the accuracy of two altimetry satellites. High-precision altimetry satellites were selected to explore the intra-year and inter-annual dynamic changes of water level in the coastal wetland at the intra-year, monthly, seasonal, and annual average water level. The results showed that the best re-setting effect was achieved in the improved threshold for the water level of coastal wetlands. By Jason-3A, the maximum determination coefficient was 0.78, the minimum RMSE was 0.35 m, and the minimum MAE was 0.28 m. By Sentinel-3A, the maximum determination coefficient was 0.87, the minimum RMSE was 0.24 m, and the minimum MAE was 0.18 m. It inferred that the high monitoring accuracy was obtained by Sentinel-3A. There was also obvious seasonality for the water level change of the coastal wetland in Beibu Gulf from 2016 to 2020. The average change of the water level was 3.37 m during the year. There was a drastic downward trend in the water level changes as a whole, with an average annual change rate of 0.005 m. The spaceborne radar altimeter provides a powerful method for monitoring the water level of coastal wetlands on a large scale, which is of great significance to the study of coastal wetland changes and ecological environment protection.