5.1 Physical properties of recession characteristics
In order to understand whether the reformed parameters a andb are still relevant to catchments characteristics, this study also analyzed their Spearman’s rank correlation. The catchments characteristics (Table 4) include catchment area, elevation, mean catchment slope, length of main channel, the mean channel slope, and BFI (baseflow index, which represents the ratio of baseflow to total streamflow). The correlation between reformed parameters and catchments characteristics are shown in Figure 7. The parameter b only had a moderate correlation with the mean channel slope, indicating that the inclination of the aquifers has an effect on the catchment drainage process. These are also consistent with the previous study (Vannier et al., 2014; Santos et al., 2014; Sánchez-Murillo et al., 2015) that assumed the baseflow coefficient be change with the aquifer slope. The parameter a had a strongly negative correlation with the BFI, a moderately positive correlation with the length of main the channel and main channel slope. Biswal and Marani (2014) and Shaw (2016) mentioned the catchment drainage networks shrink with change in the coefficient ae . In BFI, its representative of the catchment drainage condition and ability demonstrated the antecedent wet condition controls the initial streamflow condition before recession occur and the recession rate (Patnaik et al., 2015; Bart and Hope, 2014).
Here, this study also plotted a vs b to explore the regional differences in the catchment recession regimes (Figure 8). The overall result showed that parameters a and b are inversely proportional. The variability of b in the Chianan Plain was higher than in the Pingtung Plain. In the northern and southern subareas of the Chianan Plain, there was greater variability in the southern subarea. The results indicated that the variabilities may be related to the geological structure in Southern Taiwan. In addition, a more developed aquifer exists above the Pingtung Plain as compared to the Chianan Plain. The presence of marine mudstones reduces lateral aquifer connectivity in the southern subarea of the Chianan Plain. Therefore, the aquifer properties and structure can be regarded as one of the main factors causing differences in parameter b in each catchment.
Based on the above results, we can understand the reformed parameters still retained their physical properties after the impact parameterx had been added. Assuming the geological properties were unchanged, the change in a between the pre- and post-period will mainly affected by the external factors including land cover change, land use condition, climate change, etc., which were quantified to the impact parameter x . Therefore, the recession regime influenced by the overall environmental change should focus on the dynamic storage and storage-discharge relationship.
5.2 Environmental impact on dynamic storage properties
Compared dynamic storage with Δx , most of the catchments have consistent changes except Shin-Ying, Tso-Chen, and Liu-Kwei. This indicates that groundwater storage variation increases when parameterx increases, also indicating that parameter x would capture the original groundwater storage variation by inversely estimating from Q . Interestingly, more dynamic storage and temporal changes were mainly concentrated in the Pingtung Plain (Liu-Kwei, San-Ti-Men, Chao-Chou and Hsin-Pei) and the Bazhang River Basin (Chu-Kou and Chang-Pan Bridge). As mentioned in Sections 2 and 5.1, northern Chianan Plain and Pingtung Plain have great lateral aquifer connectivity and a more developed aquifer. The hydrogeological parameters estimated by Huang and Yeh (2019) also showed this significant regional difference.
In S -Q relationships, in addition to indicating that the catchment S -Q relationships become less susceptible to environmental changes, it also can be considered as the dimensionlessS-Q relationship to compare regional difference. Higher sensitivities concentrated in the central region of Southern Taiwan and the decreasing change are similar to our previous work. However, the spatial distribution was different from larger dynamic storage with the higher susceptibility due to difference in the range of streamflow contribution. Relative to the storage capacity and the drainage characteristics formed by subsurface structures and hydrogeological properties, the vertical flow process (infiltration rate, groundwater evaporation, vegetation transpiration, etc.) also cause regional differences in the recession regime with different geomorphology or climatic conditions (Brooks, Chorover, Fan, Godsey, Maxwell, McNamara, & Tague, 2015; Tashie, Scaife, & Band, 2019). In addition, our results provided the perspective for quantified environmental impact and change in sensitivities. As Cheng et al. (2017) mentioned, the increasing parameter x indicates that the environmental change caused more groundwater storage loss thereby reducing groundwater discharge. Most catchments had the decreasing S -Q sensitivity with an increase in parameter x , showing the groundwater storage loss and lower baseflow are the crisis under the environmental change. It may lead to an increase in drought events, a change in ecological habitat, and even prompt people to consume more groundwater resources.