## 5 Results and Discussion
### Effect of Individual Processing Steps on Soil Moisture Estimation
The lowest correlation is found at 0.025 m soil depths at station 9 and 5 in 2020, with R\({}^{2}\) values of 0.01 and 0.09, respectively, while the highest correlations can be found at 0.1 m soil depth at station 10 with 0.68 in 2019 and station 5 with 0.51 in 2018. Here, the uRMSE is highest in 0.025 m soil depth at station 10 in 2019 with 0.112 m\({}^{3}\)/m\({}^{3}\), while it is lowest at the same year and station but in 0.1 m soil depth with 0.044 m\({}^{3}\)/m\({}^{3}\) (Table 6). Even though this could be caused by differing soil moisture within the related soil depths, most likely it is due to corrupted measurements, as other stations do not show such a significant difference. As described in Section 5.1, the influence of the alternating incidence angles on the backscattering signal is still present in the incidence angle normalised time series, affecting the resulting soil moisture estimation. Grouped in the three different vegetation periods, the different range of in-situ measured and estimated soil moisture is displayed (Figure 10). For the Apulian Tavoliere site at 0.025 m soil depth, the estimated soil moisture range matches the observed one. However, for the Rur catchment, there was an overestimation of the minimum soil moisture, while for the Apulian Tavoliere site at 0.1 m, there is an underestimation. This suggests that the scaling from GLDAS may not be optimal but can be further improved. Furthermore, the estimated soil moisture still showed extreme low and high values due to the incidence angle still effect present in the backscatter time series. Crop-dominated stations showed significant variation in correlation and error between individual years due to the influence of incidence angle, which depends on plant geometry, plant phase, and vegetation water content, changing both between the year as well as within a year [72; 84]. In contrast, the soil moisture estimation at the meadow station (RU_C_005) is stable over the years, with only slight changes in correlation and uRMSE. Dominated by meadows with short grass vegetation, it results in a more stationary incidence angle effect during the growing period. As the incidence angle effect is in general not as prominent due  
\begin{table}
\begin{tabular}{c c c c c c c c c c c c c c} \hline \hline \multicolumn{2}{c}{**Soil Depth**} & \multicolumn{4}{c}{**0.025 m**} & \multicolumn{4}{c}{**0.1 m**} \\ \hline \hline  & \multicolumn{4}{c}{**R\({}^{2}\)**} & \multicolumn{4}{c}{**uRMSE [vol. \%]**} & \multicolumn{4}{c}{**R\({}^{2}\)**} & \multicolumn{4}{c}{**uRMSE [vol. \%]**} \\ \hline  & **TDR** & **1A** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** & **IA + FS** \\ \hline  & Station\_2 & 0.42 & 0.50 & 0.30 & 5.04 & 4.60 & 5.42 & 0.33 & 0.42 & 0.30 & 6.64 & 6.35 & 7.06 \\ \cline{2-13}  & Station\_3 & 0.35 & 0.34 & 0.29 & 5.28 & 5.77 & 5.61 & 0.28 & 0.27 & 0.36 & 5.71 & 6.09 to the short vegetation cover, for these types of landcovers, an incidence angle normalized backscattering time series from multi-orbit Sentinel-1 scenes can already estimate the soil moisture sufficiently during the whole year. The soil moisture estimation from incidence angle normalized and Fourier Series transformed time series (IA + FS) generally improves in terms of both correlation coefficient and uRMSE. The mean R\({}^{2}\) from individual CRNP stations is between 0.47 and 0.57, while the corresponding uRMSE values range between 0.045 m\({}^{3}\)/m\({}^{3}\) and 0.052 m\({}^{3}\)/m\({}^{3}\) (Table 4). The strongest improvement is observed in 2019, with a 79% increase in R\({}^{2}\) and 22% decrease in uRMSE. However, Fourier transformation led to reduced correlations in four cases, all being observed at crop dominated stations (Table 5). At the Apulian Tavoliere site, the mean R\({}^{2}\) from the individual capacitance stations at 0.025 m range between 0.16 and 0.39 with corresponding uRMSE between 0.058 m\({}^{3}\)/m and 0.081 m\({}^{3}\)/m\({}^{3}\).