“…However, these satellites provide SM observation only for the surface layer (∼top 5 cm). Different data assimilation methods have been used to retrieve the root zone SM profile from surface SM observations [27]- [30], [84] and most of these techniques are based on Kalman filtering [27], [32]- [38]. For example, Dunne and Entekhabi [31] used the ensemble Kalman smoother methods to estimate the surface and subsurface SM by assimilating L-band radio brightness temperatures.…”