*2.3. SMAP Data*

Soil Moisture Active and Passive (SMAP) is an Earth observation mission carrying an L-band radiometer and radar, which provides soil moisture and freeze–thaw data with a time-resolution of 2–3 days from NASA (https://nsidc.org/data/smap/smap-data.html (accessed on 6 November 2021)). In this study, soil moisture data (L3\_SM\_P) of SMAP L3 were used, which were obtained by retrieval using the Single Channel Algorithm (SCA) with a spatial resolution of 36 km. The time period was selected from July 2019 to December 2021. SMAP was divided into two types of data: daily 6:00 a.m. (descending orbit) and 18:00 p.m. (ascending orbit). It has been shown that the surface temperature homogenization at 6:00 a.m. is better than that at 18:00 p.m., and the retrieval accuracy is higher than that at 18:00 [27]. In this study, both descending orbit and ascending orbit data were incorporated and averaged to obtain soil moisture data of SMAP.
