FIGURE 6 Semi-variogram with strong spatial dependence during the period
of August 2019 - February 2020
Table 4 shows the spatial dependence of water table data from November
2018 until July 2019 are classified as weak spatial dependence
(>75%). A flat semi-variogram indicates weak dependence,
with most range value is 0 m, and it shows no spatial correlation
(Figure 5).
Moderate – strong spatial dependence is shown from water table data
from August 2019 until February 2020, which indicates spatial
correlation with the correlation range average within this period is
2258 m (Figure 6). From this point forward, the analysis will be
separated into two groups data: weak spatial dependence (November 2018
– July 2019) and strong spatial dependence (August 2019 – February
2020).
Cross-validation for all data is done to see the accuracy of the Kriging
map, based on the predicted and measured data (Table 5). For weak
spatial dependence, the mean error is negative, which means that the
predicted value is lower than the actual measurement. For strong spatial
dependence, the mean error is positive, which means that the predicted
value is higher than the actual measurement. But overall, all the mean
error values are close to 0, which means the predictions are unbiased.
Most of the root-mean-square standardized error values close to 1, which
indicates the standard errors are normally distributed. RMSE value for
weak spatial dependence data is higher than RMSE value for strong
spatial dependence data. It demonstrates that weak spatial dependence
has a higher variance compared to strong spatial dependence. Similar
circumstances have occurred for average standard error, where the weak
spatial dependence data has a higher value compared with the strong
spatial dependence.
TABLE 5 Cross-validation results of Kriging map for the period of
November 2018 - February 2020