Soil moisture ( SM ) is crucial for the Earth's ecosystem, impacting climate and vegetation health. Obtaining in situ observations of SM is labor-intensive and complex, particularly in remote and ...densely vegetated regions like the Amazon rainforest. NASA's soil moisture active and passive (SMAP) mission, utilizing an L-band radiometer, aims to monitor global SM . While it has been validated in areas with low vegetation water content ( VWC ) (< 5 <inline-formula><tex-math notation="LaTeX">{\text{kgm}}^{ - 2}</tex-math></inline-formula>), its efficiency in the Amazon, with dense canopies and high VWC (> 10 <inline-formula><tex-math notation="LaTeX">{\text{kgm}}^{ - 2}</tex-math></inline-formula>), is limitedly investigated due to scarce in situ measurements. This study assessed and analyzed the SMAP SM retrievals in the Amazon, employing the single-channel algorithm and adjusting vegetation optical depth (τ) and single scattering albedo (ω), two key vegetation parameters. It incorporated in situ SM observations from three old-growth rainforest locations: Tambopata (Southwest Amazon), Manaus (Central Amazon), and Caxiuana (Eastern Amazon). The SMAP SM deviated substantially from the in situ SM . However, calibrating τ and ω values, characterized by a lower τ, resulted in better agreement with the in situ measurements. This study emphasizes the pressing need for innovative methodologies to accurately retrieve SM in high- VWC regions like the Amazon rainforest using SMAP data.
