3.4. The number of required samples (NRS) for SWC
To obtain the minimum NRS to determine mean SWC in this semi- and arid region, a bootstrap resample of several plots had been simulated using Matlab script. Regardless of the initial value, the mean quickly converged to a stable value as the sample number increased with step sampling (Fig. 6a). To qualitatively determine the threshold of function-shifting, the first derivative of each fitting function was calculated in Fig. 6b. Two curve inflection points (the derivative was equal to ± 1 and ± 0.58, with corresponding slopes of the fitting curve of 45o and 30o) were marked on the graph, and four thresholds were found in this study.