A remote sensing-based area dataset for approximately 40 years that reveals the hydrological asynchrony of Lake Chad based on Google Earth Engine

As the second largest lake in Africa, the extent of Lake Chad has over 50% seasonal variations with large parts of water under aquatic vegetation. Although a great area shrinkage since the 1960s divided the lake into two parts, namely, the northern and southern basins, it still feeds millions of people from four countries around it. Given sparse in situ measurements, remote sensing with seamless spatial coverage has served for acquiring the long-term area series of Lake Chad. This study retrieves the open (unvegetated) surface water area series of Lake Chad and the total inundation area, including open water and water under macrophytes, combining multiple remote sensing data from around 1980 to 2020 with satellite-based water level and in situ observations at the Bol gauge as validation and auxiliary. Results show that the total inundation area continuously recovered at a rate of 145 km2/year from 1982 to 2020 with large annual fluctuations. Approximately two- to three-month time lags between the open surface water and the total inundation area reveal the hydrological asynchrony of Lake Chad, which can be attributed by the large area of rooted macrophytes. The overspill of the southern surface water flowing over the Great Barrier into the northern lake also has a one-month lag with the open surface water of the southern lake. The Google Earth Engine-based online application can be easily shared for use and data downloaded by the general public, and the open code can be adjusted for any other lakes worldwide.

作为非洲第二大湖泊，乍得湖的水域面积存在超过50%的季节性波动，其大片水域被大型水生植物（macrophytes）覆盖。尽管自20世纪60年代以来，该湖经历了大幅面积萎缩，被分割为南北两个盆地，但它仍维系着周边四国数百万民众的生计。鉴于原位（in situ）实测数据稀缺，具备无缝空间覆盖能力的遥感技术成为获取乍得湖长期水域面积序列的核心手段。本研究整合1980年至2020年前后的多源遥感数据、卫星水位数据以及博尔（Bol）水位站的原位观测数据作为验证与辅助数据源，反演得到乍得湖的开阔（无植被覆盖）水面面积序列，以及涵盖开阔水体与大型水生植物覆盖下水体的总淹没面积序列。研究结果显示，1982年至2020年间，乍得湖总淹没面积以每年145平方公里的速率持续恢复，但年际波动幅度较大。开阔水面与总淹没面积之间存在约2至3个月的时间滞后，这反映出乍得湖的水文异步性，该现象可归因于湖区大面积分布的扎根型大型水生植物。南部湖区水体翻越大屏障（Great Barrier）涌入北部湖区的溢流过程，与南部湖区开阔水面的变化同样存在约1个月的时间滞后。本研究基于谷歌地球引擎（Google Earth Engine）开发的在线应用程序可便捷共享以供使用，公众可直接下载相关数据，且其开源代码可针对全球任意其他湖泊进行调整适配。