Construction of the 500-m Resolution Daily Global Surface Water Change Database (2001-2016)

Surface water is the most dynamic land-cover type. Transitions between water and nonwater types (such as vegetation and ice) can happen momentarily. More frequent mapping is necessary to study the changing patterns of water. However, monitoring of long-term global water changes at high spatial resolution and in high temporal frequency is challenging. Here we report the generation of a daily global water map data set at 500-m resolution from 2001 to 2016 based on the daily reflectance time series from Moderate Resolution Imaging Spectroradiometer. Each single-date image is classified into three types: water, ice/snow, and land. Following temporal consistency check and spatial-temporal interpolation for missing data, we conducted a series of validation of the water data set. The producer's accuracy and user's accuracy are 94.61% and 93.57%, respectively, when checked with classification results derived from 30-m resolution Landsat images. Both the producer's accuracy and user's accuracy reached better than 90% when compared with manually interpreted large-sized sample units (1,000mx1,000m) collected in a previous global land cover mapping project. Generally, the global inland water area reaches its maximum (similar to 3.80x10(6)km(2)) in September and minimum (similar to 1.50x10(6)km(2)) in February during an annual cycle. Short-duration water bodies, sea level rise effects, different types of rice field use can be detected from the daily water maps. The size distribution of global water bodies is also discussed from the perspective of the number of water bodies and the corresponding water area. In addition, the daily water maps can precisely reflect water freezing and help correct water areas with inconsistent cloud flags in the MOD09GA quality assessment layer. Plain Language Summary Daily global inland surface water maps are produced from more than 1.9 million frames of satellite images for the period of 2001-2016 with a spatial resolution of 500m. From this time series of maps, we found that the inland surface water on Earth varies greatly in area within an annual cycle. It can reach more than 3.8 million square kilometers in September and reduce to approximately 1.5 million square kilometers in February. We demonstrate that (1) short-duration water bodies in arid areas, which are particularly important to life, can be detected from these daily water maps; (2) sea level rise effects on land submersion can be detected over some gentle-slope coasts like west Florida; and (3) different types of rice field use exist in the world. For example, in California, United States, rice fields are filled with water after harvest to help create a wetland environment for wild birds in the winter.