利用Jason-3/Sentinel-3A雷达高度计监测北部湾滨海湿地水位变化

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

为了评估雷达高度计监测滨海湿地水位的能力,提高其监测滨海湿地水位的测量精度,该研究以广西北部湾滨海湿地为研究区,选取2016—2020年逐日的Jason-3和Sentinel-3A雷达高度计数据,建立滨海湿地水位遥感监测模型,利用重心偏移法,阈值法,改进的重心偏移法和改进的阈值法对测高水位进行波形重定改正获取滨海湿地水位。结合水文站实测数据,定量评估不同算法提取水位的精度差异,利用年内水位变幅、月平均水位、季平均水位和年平均水位探究滨海湿地水位动态变化,利用降水量数据分析滨海湿地水位动态变化原因。结果表明,在4种重跟踪算法中,改进的阈值法重定效果最佳,Jason-3雷达高度计的决定系数最大为0.78,均方根误差(Root Mean Square Error,RMSE)最小为0.35 m,平均绝对误差(Mean Absolute Error,MAE)最小为0.28 m;Sentinel-3A雷达高度计的决定系数最大为0.87,RMSE最小为0.24m,MAE最小为0.18m,表明Sentinel-3A雷达高度计的监测精度较高。广西北部湾滨海湿地水位变化与降水量变化的相关性较高,年内水位变化较为剧烈,呈现明显的季节性,年内水位平均变幅为3.37 m,在2016—2020年间水位整体呈现下降趋势,年平均变化速率为0.005 m。星载雷达高度计为大范围的滨海湿地水位监测提供了有力手段,这对研究滨海湿地变化及生态环境保护具有重要意义。

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-3 A 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-3 A,the maximum determination coefficient was 0.78,the minimum RMSE was 0.35 m,and the minimum MAE was 0.28 m.By Sentinel-3 A,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-3 A.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.