Sustainable utilization of water resources is a critical concern in Taiwan because of the island’s environment
features including mountainous topography, rapid surface flow, and uneven distribution of rainfall in time and
space. Although groundwater resources are abundant in mountainous areas, most studies on groundwater monitoring
in mountainous environments in Taiwan have focused on deep groundwater. By contrast, high-spatial-resolution information on shallow groundwater characteristics remains lacking. To determine the dynamics of shallow groundwater in headwater catchments in Taiwan, this study installed seven groundwater wells using portable drilling equipment and constructed simple weirs to monitor surface runoff at several locations along a gully in a natural forest. The geological structure of the gully was heterogeneous and could be classified as colluvium. The depth of the groundwater table could not be estimated using geological structure information; however, it could be directly detected by boring using the portable drilling equipment. The dynamics of the groundwater varied with the locations along the gully. The deepest point of the shallow groundwater and the largest variations in the shallow groundwater table were both observed at the middle slope. Shallow groundwater pathways were divergent at the upper-middle slope where the hydrological connectivity between surface runoff and shallow groundwater was weak. By contrast, shallow groundwater pathways were convergent at the lower-middle slope where the hydrological connectivity between surface runoff and shallow groundwater was strong. The flowing pattern of shallow groundwater could be classified as hyporheic flow. This study demonstrated
that portable drilling equipment can be effectively employed for shallow groundwater observations in moun
tainous areas in Taiwan; this approach can help to clarify the mechanism of rainfall-runoff processes in headwater
catchments and to evaluate the water resources in mountainous environments.
Key Words: colluvium, hyporheic flow, portable drilling equipment, shallow groundwater.