## 3 Results  
### Validation of FDR Measurements  
#### 3.1.1 Reference Dataset  
The summary statistics of the reference dataset used for calibration are presented in Table 13. On the plots WIJ and GON, the upper FDR sensors were located in the forest floor layers (OFH) of B and C profiles, while in mineral topsoil (Ah layer) for the A profile. Hence, bulk samples for reference \(\theta_{\mathrm{v}}\) were taken in the same OFH layers (\(n\) = 47 in WIJ and \(n\) = 50 in GON) and the remaining in M01 layer of the A profiles. Local spatial variation in OFH thickness caused some reference samples to be more mineral than organic and vice versa. Therefore, we considered reference samples having a \(\theta_{\mathrm{pkop}}<\) 850 kg m\({}^{-3}\) (threshold derived from [29]) as part of OFH layers, and above this threshold as M01. On RAV and HOE, two reference Kopecky samples qualified as OFH samples, whereas none in BRA topsoil samples. Over the total calibration period raw FDR sensor output ranges from 20.5 to 34.1 \(\upmu\)s in the OFH layers, and from 17 to 41.9 \(\upmu\)s in mineral layers, covering the full range reported by the manufacturer [20]. Note that on the plots WIJ and GON, neither significant differences in period means nor ranges were found between the FDRs in the OFH and those in the mineral layer (M01) for the same site. However, the observed mean period was lower on the sandy site WIJ compared to the heavier textured GON site (Table 1). Overall, the _PA_ values are lower in the sandy soils (WIJ, RAV and BRA) than in the silty and clayey soils of GON and HOE. Within sites, the mean _PA_ values differ significantly among depth layers as shown by the 95% confidence interval of the means, which can be attributed both to specific soil layer properties and a possible sensor effect. Each sites' soil temperature was sensed in the A profile only. Mean annual soil temperature was roughly above 10 \({}^{\circ}\)C on the sandy sites, and slightly lower on the heavier textured soils, but this difference is neither significant between sites nor between soil layers within the sites for the reference dataset. However, the range is wider in the topsoil layers, especially in the forest floor (OFH) which may warm up to 25 \({}^{\circ}\)C. In subsoil layers, the amplitude of the temperature fluctuations is reduced due to the well-known damping effect. Soil bulk density, known for its significant effect on dielectric permittivity, is significantly smaller in the OFH layers compared to the mineral layers (M01-M48). In the mineral soil profile of each site, \(\rho_{\mathrm{b}}\) generally increases with depth. Lowest \(\rho_{\mathrm{b}}\) values are found in GON and highest in HOE (Retisol), but also elevated values in the Arenosols (RAV and BRA). At the WIJ and GON sites, the mean gravimetric soil water content (\(\theta_{\mathrm{m}}\)) in the OFH layers is more than three times \(\theta_{\mathrm{m}}\) in the mineral soil, clearly showing that forest floor layers can hold more water than their dry mass compared to mineral layers generally storing less than 50% of their mass. On all sites, mean \(\theta_{\mathrm{m}}\) significantly differs by layer with higher mean moisture contents in the 0-10 cm topsoil (M01) due to elevated contents of organic matter. Depending on the depth of the groundwater, perched water table or subsurface flows, mean \(\theta_{\mathrm{m}}\) can rise again in the M48 layer. Volumetric soil water content (\(\theta_{\mathrm{v}}\)) is greatest in the OFH up to 0.62 m\({}^{3}\) m\({}^{-3}\) while maximally 0.52 m\({}^{3}\) m\({}^{-3}\) in M01 layers. Again, significant differences in means are found by mineral layers within sites and between sites, justifying the need for multiple moisture sensors.