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.

乍得湖为非洲第二大湖，其水域范围存在超过50%的季节变化，大量水体被水生植被覆盖。自20世纪60年代以来，湖面大幅萎缩，将乍得湖分割为南北两个盆地，但该湖依然为周边四国的数百万民众提供生计支撑。鉴于原位观测数据稀缺，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年左右的多源遥感数据、卫星水位数据以及博尔水文站的原位观测数据作为验证与辅助资料，反演得到乍得湖的开阔（无植被覆盖）地表水面积序列与总淹没面积序列——总淹没面积涵盖开阔水域及大型水生植物覆盖下的水体。研究结果显示，1982年至2020年间，总淹没面积以每年145平方千米的速率持续恢复，但年际波动幅度显著。开阔地表水与总淹没面积之间存在约2至3个月的时间滞后，这一现象揭示了乍得湖的水文异步性，其成因可归结为大面积扎根型大型水生植物的影响。南部地表水越过大屏障（Great Barrier）涌入北部湖体的过程，相较南部开阔地表水也存在1个月的滞后。本研究基于谷歌地球引擎（Google Earth Engine, GEE）开发的在线应用可便捷共享供公众使用并下载数据，且开源代码可针对全球其他任意湖泊进行适配调整。