1. Introduction
Recession analysis proposed by Brutsaert and Nieber (1977) is usually
used to explore the relationship between groundwater storage and
streamflow. With the function between streamflow and flow variation, the
process of groundwater discharge from the aquifer to the river channel
can be characterized (Brutsaert, 2008). Due to the availability of
hydrological data, this method has generally been used to evaluate the
drainage behavior (Thomas et al., 2013; Lin and Yeh, 2017; Parra et al.,
2019) and dynamic storage (Buttle, 2016, 2018; Dralle, Karst,
Charalampous, Veenstra, & Thompson, 2018; Meriö et al., 2019; Lin et
al., 2020) that is sensitive to streamflow at the basin/catchment scale.
The main focus of this method includes analyzing the basin hydrological
conditions, spatial distribution of aquifer properties, and
geomorphology as well as channel networks of basins (Roques, Rupp, &
Selker, 2017).
It is widely used for various applications, including groundwater
hydraulic characteristics estimation (Mendoza, Steenhuis, Walter, &
Parlange, 2003; Dewandel, Lachassagne, Bakalowicz, Weng, & Al-Malki,
2003; Oyarzún et al., 2014; Arumí, Troch, Maddock, Meixner, & Eastoe,
2016; Huang & Yeh, 2019), drought detection (Stoelzle, Stahl, &
Weiler, 2013; Stoelzle, Stahl, Morhard, & Weiler, 2014), regional
low-flow analysis (Van Dijk, 2010; Beck et al. 2013), basin topographic
impact assessment, and basin river network dynamic analysis (Biswal &
Marani, 2010; Biswal & Nagesh Kumar, 2013; Biswal & Nagesh Kumar,
2014; Ward et al., 2016; Li, Zhang, Long, & Feng, 2017). Previous
studies have also shown that the impact of the co-evolution among soil,
vegetation, atmosphere, and human activities on the hydrological cycle
can help explain the effects on the basin hydrological processes.
Related research has also shown that the regional differences in basins
with different factors such as geology and land coverage can be
elucidated by the recession characteristics. (Bogaart, Van Der Velde,
Lyon, & Dekker, 2016).
Savenije, Hoekstra, and Van der Zaag (2014) suggested that human
activities have direct impacts such as river separation, reservoir and
hydraulic structure formation, pumping and return flow, in addition to
indirect impacts such as afforestation or deforestation, land use
change, and climate change on a basin hydrological system. Combining the
various influencing factors mentioned above, we can appreciate the
importance of understanding the hydrological cycle change regime and
elucidating the hydrological processes under environmental change. Some
researchers have attempted to retained or added relevant hydrological
variables in recession analysis based on the effects of the related
environmental impacts. Szilagyi,
Gribovszki, and Kalicz (2007) used a non-linear reservoir model to
evaluate catchment evapotranspiration during the flow recession period
through the lumped water balance model, comparing with the monthly
estimation evapotranspiration measured by a two-dimensional finite
element numerical model. Wang and Cai (2009, 2010a, 2010b) and Wang
(2011) included groundwater pumping, return flow of agricultural
irrigation, and bedrock leakage as variables in the recession process in
agricultural and highly-urban basins. The results showed that the
streamflow data provides an excellent explanation of the direct impacts
on the catchment drainage behaviors. Thomas, Vogel, Kroll, and
Famiglietti (2013) analyzed the importance of the baseflow recession
model by incorporating human withdrawals and explored the deviations
between catchment recession characteristics with and without human
withdrawals. Cheng et al. (2017) used the paired-catchment approach and
recession analysis considering changes in the vegetation coverage,
thereby quantifying the impact of vegetation change on groundwater
discharge regime. The results improved the predictability of the impact
of vegetation change on catchment hydrological process. Recently, Ploum,
Lyon, Teuling, Laudon, and van der Velde (2019) explored the effects of
snowmelt caused by global warming on the runoff processes and flow paths
through seasonal storage-discharge dynamics with linear and non-linear
relationships.
In recent years, due to climate change, wet and dry years alternate more
frequently in Taiwan. These changes in climate pattern include an
increase in rainfall gap, rainfall intensity, temperature, and extreme
events. The Water Resources Agency (2016) analyzed the rainfall trend in
Taiwan using five general circulation models (GCM), evaluating the worst
rainfall. The results showed that future rainfall and streamflow will
gradually decrease in Taiwan basins, especially in Southern Taiwan. The
main risk associated with this pattern are rising temperature, reduction
of surface water resources, decline of reservoir water storage capacity,
and increase in agricultural water demand. Groundwater is a relatively
stable water resource due to the faster runoff with higher terrain and
reservoir sedimentation in Taiwan. However, groundwater resource
management has received less attention due to monitoring challenges. It
is often neglected in developing countries, which leads to the excessive
use of groundwater having various environment affects (Famiglietti,
2014). In Taiwan, groundwater pumping accounts for one-fifth of the
available water resources (Water Resources Agency, 2014). A portion of
the groundwater also provides streamflow during no-rainfall periods to
maintain ecological habitats within the basin. However, most researchers
have focused on surface hydrology such as runoff and evapotranspiration
in different climate scenarios to explore the geological structural
characteristics and rainfall infiltration regime in the basin. As a
result, groundwater affected by climate change and human activities is
less frequently discussed (Zhang, Brutsaert, Crosbie, & Potter, 2014;
IPCC, 2014).
With industrial development and increase in population, the environment
in Taiwan has undergone many changes. Land use in flat lands has been
saturated, and urbanization has significantly increased areas with
impermeable surfaces. Improper development of hillslopes coupled with
the effects of earthquake and typhoons has resulted in increasing soil
and water conservation problems. In addition, groundwater depletion in
coastal areas that results from the over-pumping of groundwater by
aquaculture and agriculture, has led to land subsidence with permanent
aquifer shrinkage. Due to the complex interaction of different
environmental impacts affecting groundwater–runoff process, it is
difficult to assess the impact of varying environmental changes.
Following Yeh and Huang’s (2019) research, we focused on the catchments
of Southern Taiwan by adding more available stations in order to discuss
regional differences. The objectives of this study were (1) using
low-flow recession analysis to evaluate the temporal and spatial changes
in recession characteristics by considering environmental impacts, (2)
exploring relationship between catchment storage-discharge dynamics and
quantified impact, and (3) comparing the impact with change in
vegetation cover as a simple land use to understand how the external
factors affect catchment drainage process.