## 6 Discussion on Best Results Achieved
By observing \(\Delta\sigma^{\circ}\) due to vegetation variations (i.e., changes in _LAI_ and \(F_{\rm v}\)) \(\Delta\sigma^{\circ}{}_{\rm pq}\) are higher at L (\(\sim\)14 dB) rather than at C frequency (\(\sim\)10 dB); this is probably due to the structure of sugar beet characterized by very large leaves. Moreover, as confirmed by other studies, \(\sigma^{\circ}{}_{\rm HV}\) and \(\sigma^{\circ}{}_{\rm VH}\) (not shown here) behave quite similarly according to the reciprocity principle, which for most natural media assumes a symmetric scattering matrix [59]. The correlation analysis highlights that over bare soils, \(\sigma^{\circ}\) vs. \(m_{\rm v}\) are significant and characterized by high \(r\) at all channels with exceptions of C\({}_{\rm HV}\) and C\({}_{\rm VV}\). The highest \(r\) and the lowest MAE were found at L frequency. Whereas no significant \(\sigma^{\circ}\) vs. _h'_ correlations were achieved. Over vegetated areas, since _h'_ was measured below canopy, no significant \(r\) values have been found as \(\sigma^{\circ}\) over dense vegetation is mainly ruled by: (i) surface water contents (\(m_{\rm v}\) and canopy water content) and (ii) surface roughness (correlated with biomass variables, including _LAI_ and \(F_{\rm v}\), and vegetation structure). _Quasi_-significant and significant moderate to strong correlations (i.e., 0.4 < \(r\) < 0.8) characterized \(\sigma^{\circ}\) vs. \(m_{\rm v}\) at all channels with the exception of C\({}_{\rm VV}\). The highest \(r\) values were found at L frequency probably due to the higher ability of longer waves to penetrate the canopy layer. Very strong correlations (i.e., \(r\) > 0.8) were found for \(\sigma^{\circ}\) vs. \(F_{\rm v}\) at all frequencies and polarizations; highest \(r\) characterized \(\sigma^{\circ}\) vs. \(F_{\rm v}\) at X\({}_{\rm HH}\), C\({}_{\rm HV}\) and L\({}_{\rm HV}\) comparisons. Strong to very strong \(r\) characterized \(\sigma^{\circ}\) vs. _LAI_ comparisons with the highest values at C\({}_{\rm HV}\) and L frequencies. All these results highlight that, on the average, \(\sigma^{\circ}\) at L bands are more suitable than others to infer SVV over both bare and vegetated areas. It has been observed that, usually, correlations showing the highest \(r\) were also characterized by the lower MAE (although this is not a general rule). Over bare soils and for all channels, lower MAE were found for \(\sigma^{\circ}\) vs. \(m_{\rm v}\) (MAE was \(\approx\)0.6 dB on average) with respect to the vegetated soils (\(\approx\)1.3 dB). Lower MAE characterized both \(\sigma^{\circ}\) vs. \(F_{\rm v}\) and \(\sigma^{\circ}\) vs. _LAI_ (on average \(\approx\)0.7 dB and \(\approx\)1 dB, respectively) if compared to \(\sigma^{\circ}\) vs. \(m_{\rm v}\). Finally, lowest MAE were found at L frequency for \(\sigma^{\circ}\) vs. \(m_{\rm v}\) (bare soils) and for \(\sigma^{\circ}\) vs. \(F_{\rm v}\) and \(\sigma^{\circ}\) vs. _LAI_ (vegetated plots). Over bare soils, the inter-channel correlation analysis (Table 13) revealed three _quasi_-significant and very strong \(r_{\rm s}\) vs. _h'_ correlations; thus, it was observed a remarkable improvement with respect to the single frequency approach. However, no significant \(r_{\rm s}\) vs. \(m_{\rm v}\) correlations were found, meaning that all channel pairs were useless to assess \(m_{\rm v}\) by means of this approach. Improvements with regards to the single regression approach (Table 13, highlighted in red) were observed in several cases; the dual-frequency approach allowed pointing out higher correlations (Table 13, bolded red values) if compared to the single-frequency approach. Remarkably, \(r_{\rm s}\) seems to be negatively correlated with SVV with some exceptions (see Tables 7-11). These exceptions are probably linked to the vegetation structure (as most of them were found for \(F_{\rm v}\) and _LAI_ for the wheat plot); however, further analyses are required to corroborate this hypothesis.