The effect of hydrological conditions and bioactivities on the spatial and temporal variations of streambed hydraulic characteristics at the subtropical alpine catchment

The endangered Formosan landlocked salmon that habitats only in Chichiwan Creek has been successfully rehabilitated in one of its tributaries, Yusheng Creek. However, the stream fragmentation, no surface streamflow, seriously reduced the salmon population, hampering the restoration work. The surface water-groundwater (SW-GW) interactions in a 500-m reach were examined by installing 6 monitoring wells at 3 sites to record temperatures and groundwater levels at every 15-min for two months. The automatic estimator of VFLUX and numerical model of VS2DH are used to quantify the streambed hydraulic characteristics along the reach. It is found that the entire study reach is a losing stream and the fragmented reaches have significantly higher streambed vertical hydraulic conductivity (VK) compared to the perennial one. The streambed VK at the up-stream perennial reach increased by about double after a rainfall event, probably resulting from the removal of fine grain in the streambed by the rainfall event. Although the fragmented reaches reflowed after the event, streambed VK at the mid- and down-stream fragmented reaches decreased by about 75% and 50%, respectively, compared to the pre-fragmented period. The clogging layers formed by the remains of dead diatom deposited in the streambed during the fragmented period were very likely responsible for the decreased streambed VK. The spatial and temporal variations of streambed VK influenced by hydrological conditions and even bioactivities within such a short distance reveal the complexity of SW-GW interactions. Further studies about the forming processes of clogging layer and its effect on the hydraulic characteristic should be conducted to create a more holistic and reliable picture of interaction processes.