Soil moisture plays an important role in the global water cycle and has an important impact on weather and climate, energy fluxes at the land surface, hence, the accurate estimation of this state variable is important. In this work, the potential of using surface soil moisture measurements (e.g. satellite soil moisture) to retrieve initial soil moisture profile will be explored in a synthetic study, using a reduced-adjoint variational data assimilation (hereafter RA- VDA) and a nonlinear 1D-soil water model (HYDRUS-1D). The proposed RA-VDA applies the Proper Orthogonal Decomposition (POD) technique to approximate the adjoint model in the reduced space. The accuracy and performance of the proposed RA-VDA method is illustrated with different synthetic experiments in a nonlinear physical model.