| 摘 要： |
The extent of surface water has been changing significantly due to climatic change and human activities. However, it is challenging to capture the interannual changes of inland water bodies due to their high seasonal variation and abrupt change. In this paper, a global annual surface water cover frequency dataset (GLOBMAP SWF) was generated from the MODIS land surface reflectance products during 2000-2020 to describe the seasonal and interannual dynamics of surface water. Surface water cover frequency (SWF) was proposed as the percentage of the time period when a pixel is covered by water in a year. Instead of determination of the water directly, the SWF was estimated indirectly by identifying land observations among annual clear-sky observations to reduce the influence of clouds and variability of water bodies and surface background characteristics, which helps to improve the applicability of the algorithm for different regions across the globe. The generated dataset shows better performances for frozen water, saline lakes, bright surfaces and regions with frequent cloud cover compared with the two high-frequency surface water datasets derived from MODIS data, and it captures more intermittent surface water but may underestimate small water bodies when compared with two high-resolution datasets derived from Landsat data. Compared with the high-resolution SWF maps extracted from Sentinel-1 data in eight regions that cover lakes, rivers and wetlands, the R-2 reaches 0.46 to 0.97, RMSE ranges from 7.24% to 22.62 %, and MAE is between 2.07% and 7.15 %. In 2020, the area of global maximum surface water extent is 3:38 x 10(6) km(2), of which the permanent surface water accounts for approximately 54% (1:83 x 106 km2), and the other 46% is intermittent surface water (1:55 x 10(6) km(2)). The area of global maximum and permanent surface water has been shrinking since 2001, with a change rate of 7577 and 4315 km(2) yr(-1) ( p < 0 :05), respectively, while the intermittent surface water with the SWF above 50% has been expanding (1368 km(2) yr(-1), p < 0 :01). This dataset can be used to analyze the interannual variation and change trend of highly dynamic inland waters extent with consideration of its seasonal variation. The GLOBMAP SWF data are available at https://doi.org/10.5281/zenodo.6462883 (Liu and Liu, 2022). |
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