## 4 Discussion  
### Attainable Level of Predictive Accuracy  
One of the fundamental questions of this study is the level of predictive accuracy, as indicated by the RMSPE, one can achieve using the CS616 sensor for \(\theta_{\rm v}\) measurement in various soil types compared to reference gravimetric sampling. [PERSON] et al. [11] estimated the RMSPE of the reference method to maximally 0.01 m\({}^{3}\) m\({}^{-3}\). The user manual [20] of the CS616 mentions that the factory standard calibration is accurate to \(\pm\)0.025 m\({}^{3}\) m\({}^{-3}\) in the \(\theta_{\rm v}\) range of 0\(-\)0.50 m\({}^{3}\) m\({}^{-3}\) valid for soils with low bulk electrical conductivity \(\leq\)500 uS cm\({}^{-1}\) and bulk density less than 1550 kg m\({}^{-3}\). This conforms with all forest soil layers under study. In Table 10 we compiled RMSPE values obtained with standard and revised functions in various studies for coarse to fine textured soils and organic layers using the CS616 sensors. From all mentioned studies it is clear that recalibration or correction for the specific soils significantly reduces the overall prediction error. The standard function provided by the manufacturer [20] generally performs best for sandy samples [1,10], but as clay content increases, larger prediction errors are found and soil-specific recalibration is absolutely required [1,3,4,6,7,9,11]. Most recalibrations or corrections are performed in the laboratory, since field-based calibration is challenging to perform for many users [10]. The RMSPE quality criteria set by [4] for a mean bias \(\pm\) 0.02 m\({}^{3}\) m\({}^{-3}\) and RMSE \(<\) 0.035 m\({}^{3}\) m\({}^{-3}\) or a RMSE \(<\) 0.30 m\({}^{3}\) m\({}^{-3}\) for forest soils by [18] are rarely met, even after (re)calibration or correction. Furthermore, revised calibration functions developed under controlled but limited laboratory conditions, are not always appropriate for field conditions [1,5]. In 2008, an \begin{table}
\begin{tabular}{c c c c c c} \hline \hline  & & \multicolumn{4}{c}{**RMSPE (m\({}^{3}\) m\({}^{-3}\))**} \\ \hline \multirow{2}{*}{**Type of Soil**} & \multirow{2}{*}{**USDA Texture**} & **Standard** & \multirow{2}{*}{**Revised Function**} & \multirow{2}{*}{**Error Reduction**} & \multirow{2}{*}{**Reference**} \\  & & **Function** & & & \\ \hline Sandy & Sand & 0.071 & 0.04 & \(-\)44\% & This study \\  & Sand & 0.017 & 0.006 & \(-\)65\% & [PERSON] et al. 2020 \\  & Sand & 0.058 & 0.016 & \(-\)72\% & [PERSON] et al. 2013 \\  & Loamy sand & 0.068 & 0.035 & \(-\)49\% & This study \\  & Loamy sand & 0.023 & 0.021 & \(-\)9\% & [PERSON] et al. 2020 \\  & Loamy sand & 0.034 & 0.004 & \(-\)88\% & Varble \& Chavez 2011 \\  & Sandy loam & 0.084 & 0.046 & \(-\)45\% & This study \\  & Sandy clay loam & 0.039 & 0.012 & \(-\)69\% & [PERSON] et al. 2013 \\  & Sandy clay loam & 0.049 & 0.034 & \(-\)31\% & [PERSON] et al. 2013 \\  & Sandy clay loam & 0.156 & 0.043 & \(-\)72\% & [PERSON] et al. 2013 \\  & Sandy clay loam & 0.147 & 0.021 & \(-\)86\% & Varble \& Chavez 2011 \\ Loams & Loam & 0.102 & 0.052 & \(-\)49\% & This study \\  & Silt loam & 0.064 & 0.029 & \(-\)55\% & This study \\  & Silt loam & & 0.039 & \(-\) Udawatta 2011 \\  & Silt & 0.135 & 0.043 & \(-\)68\% & This study \\  & Silty clay loam & 0.962 & 0.344 & \(-\)64\% & [PERSON] et al. 2013 \\  & Silty clay loam & 0.04 & \(-\)04 & Udawatta 2011 \\  & Clay loam & 0.157 & 0.043 & \(-\)73\% & [PERSON] et al. 2013 \\  & Clay loam & 0.289 & 0.021 & \(-\)93\% & [PERSON] \& [PERSON] 2011 \\ Clay & Silty clay & & 0.029 & \(-\) Udawatta 2011 \\  & Clay & 0.207 & 0.076 & \(-\)63\% & This study \\  & Clay (heavy Clay) & 0.05\(-\)0.15 & 0.003\(-\)0.012 & \(-\)92\% & [PERSON] 2000 \\  & Clay & 0.169 & 0.05 & \(-\)70\% & [PERSON] et al. 2013 \\ Organic & Organic & 0.218 & 0.05 & \(-\)77\% & This study \\  & Organic & 0.179 & 0.06 & \(-\)66\% & [PERSON] et al. 2013 \\ All soil types & & 0.094 & 0.045 & \(-\)52\% & This study \\ \hline \hline \end{tabular}
\end{table}