5.3 Comparing the quantified impact Δx with land use change
To understand the correlation between parameter x and land use change, we calculated the Normalized Difference Vegetation Index (NDVI) as a simple land use change so as to realize the related response mechanism. The NDVI is a numerical indicator used for assessing vegetation coverage in a target area through the reflection of red and near-infrared. Here we selected Landsat-5 satellite imagery from November to April in three periods (1988-1989, 2000-2001, and 2008-2009), as shown in Figure 9. Most of Δx in Southern Taiwan increased after 2000 which was consistent with changes in vegetation except for Chu-Kou, Chang-Pan Bridge, Hsin-Shih, and Liu-Kwei. Although the changes may actually contain various factors, the impact on groundwater storage and streamflow can be clearly identified. High consumption rate of water use showed urbanization reduces low flow and increase the flow variation through groundwater pumping (Wang & Cai, 2009,2010). Large bedrock leakage from storage to deep groundwater caused the dynamic storage loss (Wang, 2011). Cheng et al. (2017) demonstrated the increasing parameter x which indicates that the increase in vegetation cover results in lower groundwater storage as a result of strong canopy interception and transpiration. Ploum, Lyon, Adriaan, Laudon, and van der Velde (2019) suggested that more snowmelt infiltration lead to nonlinear S-Q relationship which is similar to the effect on the recession curve when parameter x decreases. This exhibits that catchments in Southern Taiwan mostly have increasing groundwater loss and decreasing baseflow owing to environmental changes. Due to the importance of baseflow, the problems of water resources allocation and management in these regions require attention under the environmental change.
The inconsistent results with Δx can be explained by the local land use which may affect the catchment drainage process. According to the previous reports, fruit trees and agriculture lands are present along the upstream bank of the Bazhang River (Chu-Kou and Chang-Pan Bridge). Therefore, groundwater drainage from the aquifer of the river bank may increase groundwater storage due to the return flow of agricultural irrigation (Δx < 0). There are fewer human activities upstream of the Laonong River (Liu-Kwei), however the large surface area exposure caused by erosion or collapse with high-intensity rainfall events (such as typhoons) on agricultural and forestry slope areas may result in an increase in rainfall infiltration (Δx< 0). Upstream of the Yanshui River (Hsin-Shih), the agricultural areas with better permeability were gradually replaced with dense industrial and residential areas that reduced the groundwater storage with the lower rainfall infiltration (Δx > 0) (Water Resources Agency, 1986, 2000b, 2007). However, there is still a critical issue regarding how to separate the environmental change impacts into unnatural (e.g. human activities, land use change) and natural (e.g. climate change) factors. It is necessary to identify whether the effects of climate change and human activities on catchment hydrological processes overlap or are offsetting. We suggest that future work of the study should improve or incorporate relevant researches to solve related problems inherent for more exploration in detail.