4.2 Dynamic storage
Dynamic storage Sd represents the maximum range of groundwater storage that discharges into the river channel.Sd ranges from 2.82 to 139.96 mm in the pre-period, and from 3.75 to 101.61 mm in the post-period. Most catchments have an increasing Sd , and only Chang-Pan Bridge, Chu-Kou, Tso-Chen, and Chao-Chou have a decreasingSd , as shown in Figure 4b. To link with the aquifer properties for Sd , we performed a Spearman rank correlation analysis between the Sdand BFI (baseflow index that represents the ratio of baseflow to total streamflow) in the pre-period as BFI represents the integral state with the catchment streamflow type and aquifer properties (Eckhardt, 2005, 2008). Baseflow were calculated using Wittenberg’s (2003) baseflow separation that inverting the recession curve equation for the nonlinear reservoir. Figure 5 shows a significantly strong correlation with BFI, indicating the relevance between Sd and regional geological structures. In addition, the significant groundwater storage variations mean that the groundwater–runoff process in these regions is more susceptible to environmental impact due to the presence of aquifers with greater permeability and connectivity.
4.3 Storage–discharge (S–Q) relationship
Storage-discharge sensitivity (ε ) indicates whether the catchment storage-discharge behavior is susceptible to environmental impacts (e. g., climate change, human activities). Following Yeh and Huang’s (2019) analysis for storage-discharge sensitivity in basins of Taiwan, we reproduced and focused on Southern Taiwan over two periods. The results showed that the S -Q sensitivities in southern Taiwan ranged from 0.026 to 4.048 mm-1 during the pre-period and from 0.032 to 2.053 mm-1 during the post-period. Regarding the large variability in the streamflow ranges among the catchments and the main influence on low streamflow, this study chose low flow Q85 (Qi , iis the percentage of exceedance probability) as representative, and the corresponding ε were divided into five orders. The regional distribution of the sensitivities in the pre- and post-period were shown in Figure 6. In the first order (ε = 1-10 mm-1), there are Chun-Huei Bridge, Tso-Chen, Chung-Te Bridge, Liu-Kwei, and San-Ti-Men in the pre-period and only AL in both periods. The catchments with higher sensitivities were evenly distributed, and most of the catchments declined during the post-period (Figure 4c). There was also no expected increase or decrease in the sensitivities from upstream to downstream in the basins. However, numerically, the higher sensitivities were mainly concentrated in the Liu-Kwei and San-Ti-Men, Tso-Chen, Yu-Tien, A-Lien and Chung-Te Bridge.