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.