1. Introduction . Soil moisture (SM) is an important state variable that significantly affects the water cycle, ecosystem, and energy exchange between the land and the atmosphere. SM information is important in different fields, such as agriculture, meteorology, hydrology, weather, and evapotranspiration forecasting [ 1 , 2 , 3 , 4 , 5 , 6 , 7 ]. In recent decades, in the large-scale domain, the development of remote sensing technology has provided more opportunities. Specifica...