As a result, SAR measurements are very sensitive to soil roughness, which in agricultural fields is affected by the characteristics of tillage [10-21]. Consequently, the parameterization of surface roughness and its spatial variability can pose major problems for soil moisture retrieval [5,19,22]. As such, accurate soil moisture retrieval with single-frequency, single incidence angle, single-pass SAR imagery is not possible without a priori soil roughness information [6]. Furthermore, if the soil is vegetated, additional information is needed with respect to the vegetation parameters (such as fresh biomass, canopy structure, ��) in order to retrieve soil moisture.
The backscattered signal from a bare soil depends on a combination of factors, including radar properties (frequency, polarization), surface characteristics (dielectric constant of the soil, and by consequence soil moisture, and surface roughness), and the incidence angle of the incoming microwave [3,15]. Different models have been proposed that relate the dielectric constant to the soil moisture content. For soil moisture retrieval studies, the following models are mainly used: the polynomial expressions fitted by Hallikainen et al. [23] and the semi-empirical four-component mixing model developed by Dobson et al. [24]. The latter model, valid for frequencies larger than 4 GHz and smaller than 18 GHz, was further extended for the 0.3 to 1.3 GHz range by Peplinsky et al. [25,26].
With respect to soil moisture retrieval, one of the first studies, carried out by Ulaby and Batlivala [27] found that the optimal radar configuration consists of Brefeldin_A a co-polarized (HH or VV) sensor operating at C-band at a 7�� to 15�� incidence angle.
For this configuration, the sensitivity of the backscattering coefficient to soil roughness is minimized. At higher incidence angles, the radar return was found to be much more sensitive to surface roughness [12,28-30]. For cross-polarizations, some studies suggested a larger sensitivity to soil moisture [31] and reduced roughness effects [32,33], however, the results of these studies Carfilzomib were inconsistent [34]. According to Holah et al. [30], the HH and HV polarizations are more sensitive to soil roughness than the VV polarization.
These findings with respect to the co polarizations were not confirmed by Baghdadi et al. [35] when studying an assembled database of ERS-2, RADARSAT-1 and ENVISAT data. They discovered that the sensitivity of the radar signal to soil moisture was not very dependent on polarization.Soil moisture retrieval from sensors characterized by a shorter wavelength than C-band is hydrologically less interesting due to the small penetration depth of the microwaves [3].