GLOBMAP SWF: a global annual surface water cover frequency dataset during 2000–2020 for change analysis of inland water bodies

Abstract. 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. Regional analysis demonstrates that our estimation results show better performances on frozen water, saline lake, bright surface and cloud-frequent regions compared with the two high-frequent surface water datasets derived from MODIS data. Compared with the high-resolution SWF maps extracted from Sentinel-1 data in four regions that cover lake, river and wetland, the R2 reaches 0.83 to 0.97, RMSE is ranging from 7.24 % to 11.28 %, and MAE is between 2.07 % and 7.15 %. In 2020, the area of global maximum surface water extent is 3.38 million km2, of which the permanent surface water accounts for approximately 54 % (1.83 million km2) and the other 46 % is intermittent surface water (1.55 million km2). The area of global maximum and permanent surface water has been shrinking since 2001, with a change rate of -7577 km2/yr and -4315 km2/yr (p < 0.05), respectively; while the intermittent surface water with the SWF above 50 % has been expanding (1368 km2/yr, 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).