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.