Coupling of runoff processes and sediment transport in mountainous watersheds underlain by different sedimentary rocks

Runoff and sediment transport were observed in two mountainous watersheds with different lithologies. One watershed is underlain by chert and the other by sandstone and shale. The chert watershed consists of bedrock cliffs and shallow regolith (0–2 m), whereas the sandstone–shale watershed is covered with a deeper regolith (1–5 m). In the sandstone–shale watershed, a large spring located in the channel head contributed to storm runoff. Tensiometric response in regolith above the spring showed negative pressure head throughout storm runoff, thus implying that the subsurface flow in bedrock predominated even during storm runoff. In the chert watershed, the runoff coefficient was larger than in the sandstone–shale watershed. Tensiometric response in regolith showed positive pressure head during storm runoff, indicating generation of rapid subsurface stormflow. Concentration of subsurface flow from side slopes increased peak streamflow in the downstream direction. In these two watersheds, different subsurface flowpaths, including bedrock fractures, facilitated different modes of sediment transport. Groundwater outflow at the spring caused seepage erosion in the sandstone–shale watershed, whereas peak streamflow transported bedload supplied by rockfall in the chert watershed.