Monitoring Daily All-Sky Evapotranspiration Over the East Asian Continent Using Multi-Channel Passive Microwave Measurements From Fengyun-3B Satellite of China

Abstract

Terrestrial evapotranspiration (ET) modulates energy and water cycles of land-atmosphere continuum. Satellite daily ET estimation under all sky (both clear and cloudy sky) remains challenging due to the difficulty in detecting vegetation dynamics under cloud cover. In this study, we utilized the microwave land surface Emissivity Difference Vegetation Index (EDVI) at X- and K-bands, which was retrieved from Chinese Fengyun-3B satellite Microwave Radiation Imager (MWRI) measurements under all sky conditions, to derive daily ET (ET_FYEDVI) over East Asia (80–145°E, 10–55°N) from 2016 to 2018, in combination with multi-source satellite all-sky radiation and ERA5 climatic data. The Fengyun-3B EDVI was used to represent the key vegetation characteristic for allocating total available energy and constraining canopy conductance. ET_FYEDVI was validated at multiple spatiotemporal scales, against daily in situ measurements from eight flux sites and annual water balance method at nine river basins. Site-scale validations showed that ET_FYEDVI generated overall good accuracy (R² = 0.78, bias of −0.1 mm day⁻¹) across forest, grass, crop, and desert sites. Basin-scale evaluations further demonstrated the stable and small bias (R² of 0.96, bias of −22.2 mm year⁻¹) in annual ET_FYEDVI from southern humid to arid inland basins. This accuracy was comparable or better than those of four global satellite ET products. More importantly, ET_FYEDVI produced low and stable accumulative bias under increasing cloud conditions, and characterized the spatiotemporal variability in continental ET even under heavy cloud cover (>90%) conditions. This study marks the first attempt to use Fengyun-3 satellite passive microwave measurements to produce continental-scale ET, showing great potential for mapping spatiotemporally continuous global daily ET.