Mapping Spatially Seamless Fractional Vegetation Cover over China at a 30-m Resolution and Semimonthly Intervals in 2010–2020 Based on Google Earth Engine

Fractional vegetation cover (FVC) is a critical biophysical parameter that characterizes the status of terrestrial ecosystems. The spatial resolutions of most existing FVC products are still at the kilometer level. However, there is growing demand for FVC products with high spatial and temporal resolutions in remote sensing applications. This study developed an operational method to generate 30-m/15-day FVC products over China. Landsat datasets were employed to generate a continuous normalized difference vegetation index (NDVI) time series based on the Google Earth Engine platform from 2010 to 2020. The NDVI was transformed to FVC using an improved vegetation index (VI)-based mixture model, which quantitatively calculated the pixelwise coefficients to transform the NDVI to FVC. A comparison between the generated FVC, the Global LAnd Surface Satellite (GLASS) FVC, and a global FVC product (GEOV3 FVC) indicated consistent spatial patterns and temporal profiles, with a root mean square deviation (RMSD) value near 0.1 and an R2 value of approximately 0.8. Direct validation was conducted using ground measurements from croplands at the Huailai site and forests at the Saihanba site. Additionally, validation was performed with the FVC time series data observed at 151 plots in 22 small watersheds. The generated FVC showed a reasonable accuracy (RMSD values of less than 0.10 for the Huailai and Saihanba sites) and temporal trajectories that were similar to the field-measured FVC (RMSD values below 0.1 and R2 values of approximately 0.9 for most small watersheds). The proposed method outperformed the traditional VI-based mixture model and had the practicability and flexibility to generate the FVC at different resolutions and at a large scale.