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. 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 R2 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 3.38×106 km2, of which the
permanent surface water accounts for approximately 54 % (1.83×106 km2), and the other 46 % is intermittent surface water (1.55×106 km2). The area of global maximum and permanent surface water has been
shrinking since 2001, with a change rate of −7577 and −4315 km2 yr−1 (p<0.05), respectively, while the intermittent surface
water with the SWF above 50 % has been expanding (1368 km2 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).