Extended alpha approximation method for the retrieval of soil moisture under dynamic vegetation by multi-incidence angle sentinel-1

The retrieval of (almost) daily soil moisture from C-band Sentinel-1 Synthetic Aperture Radar ( SAR) records is still influenced by incidence angle effects and vegetation dynamics. In this study we present a method to reduce the effects of both methods on the alpha approximation methods, a time series approach assuming changes in backscattering signals between two consecutive observations are related to a change in soil moisture. By implementing a Fourier series for incidence angle normalization and a linear regression to co-polarized backscatter for a vegetation adaption, the alpha approximation method has been extended to gain high temporal resolution soil moisture time series. The approach was tested in the Rur catchment, Germany and the Apulian Tavoliere, Italy.