2. Study Area
Southern Taiwan covers an area of approximately 10,002 km2, representing 28% of the total area of Taiwan, as shown in Figure 1. The climate is classified as a subtropical monsoon climate with distinct wet and dry seasons. As a result of the plum rain and typhoon season, rainfall is concentrated during summer and autumn (May to September). During winter and spring (October to April), the north-south Central Range blocks the northeast monsoon, resulting in relatively less rainfall. According to historical reports (Water Resources Agency, 2015), the average annual rainfall in Southern Taiwan is 2,527 mm with the ratio of the rainfall between dry and wet season is 1:9; the average streamflow is about 17.5 billion m3/year across all regions in Taiwan.
With the Kaoping River basin serve as a boundary, Southern Taiwan can be divided into the Chianan Plain and the Pingtung Plain (including the Kaoping river basin). As a result of the Chianan Plain geological structure, different conditions exist between the northern and southern region with the Yanshui River as a boundary. The northern region is a sedimentary basin that contains marine and continental deposit cross-bedding with great lateral aquifer connectivity formed by past sea level changes. As a consequence of tectonic movements, the southern region contains different types of sedimentary basins, with several basement areas composed of marine mudstones which divide the region into several independent zones with poor lateral aquifer connectivity. The upstream catchment area of the Pingtung Plain is mainly composed of interbedded sandstone and shale with lower permeability. The middle and downstream river beds, formed on a platform of modern alluvial deposits with gravel and sand, are highly permeable. The thick aquifer in the Pingtung Plain covers the entire area with an aquifuge that is significantly smaller than aquifer in the southern region (Central Geological Survey, 2012; Water Resources Agency, 2017).
An overview of water use in Southern Taiwan showed that only 20% of the available water resources generated by rainfall (about 19.57 billion tons/year) can be used as a water supply source including 9% from reservoir storage, 5% from river intake, and 6% from groundwater pumping. Steep slopes and flash streams cause water to quickly discharge into the ocean while evapotranspiration also accelerates water losses. Total water consumption is comprised of 20% domestic, 12% industrial, and 68% agricultural uses. The efficiency of water use in the region is low due to its fragile geological structure leading to lower available water resources stemming from rainfall and the difficulty in reservoir construction. According to the results in the report, water resource shortages in Southern Taiwan will become increasingly severe in the worst-case scenario of future rainfall predictions. (Water Resources Agency, 2016).
This study selected 17 hydrological stations (Table 1, seven of which were used the same as Yeh and Huang (2019)) in Southern Taiwan in order to analyze catchment storage-discharge dynamics under environmental change. According to Lu, Cho, Lee, Lee, and Lin (2012) and Wu et al. (2010), temperature and rainfall trends from 1911-2009 indicate the emergence of a different climate pattern around the year 2000. After 2000, the decadal oscillation fluctuation reduced while changes in the tropical cyclone tracks resulted in an increase in the occurrence of typhoons in Taiwan. Liu, Chen, and Chu (2008) analyzed the long-term changes in rainfall from 1961-2005 and found that, compared with the 1960s, rainfall after 2000 increased significantly (by 200%) while the number of rain days with varying rainfall levels decreased. In light of these findings, this study collected streamflow data from 1986 to 2015. The data was divided into two time periods 1986 – 2000 and 2001 – 2015. The periods were used to explore spatial and temporal changes in low-flow characteristics, dynamic groundwater storage and groundwater flux under environmental change before and after 2000.