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.