6. Conclusion
This study considered the environmental impact in low-flow recession analysis, exploring the changes in low-flow characteristics, dynamic storage, and storage-discharge relationships in order to understand the effect of groundwater loss or gain on the catchment recession regime. The results showed decreased low-flow characteristics in most catchments which represent the lower flow variation owing to environmental change. In addition, the regional distribution of low-flow characteristics and dynamic storage showed regional differences in aquifer properties, which is consistent with previous work. The majority of the dynamic storage increased in the post-period, indicating that environmental change ultimately led to increased groundwater consumption by increased vegetation cover, groundwater pumping or other possible factors. It also causes a decrease in storage-discharge sensitivities with lower streamflow. This highlighted a critical problem where frequent drought events may occur during dry periods due to a lower streamflow generated from groundwater. To understand whether the quantifying environmental change is consistent with changes in land use, this study calculated the NDVI changes for comparison. As expected, both changes were the same, and only a few catchments were inconsistent. These inconsistent cases can be explained by the possible impact factors (including return flow of agricultural irrigation, increase in industrial land use, surface erosion by natural hazards, and others) from local land use. In conclusion, this study demonstrated that quantifying environmental impacts through the recession method can help to clarify current changes in groundwater and related processes from environmental co-evolution. Different from our previous research that focused on S-Qsensitivity and its change, this study can compare regional differences and its changes over time by quantified environmental impact. Future challenges lie in the development of methods or modeling for assessing an impact of specific factor with a physics-based description.