Hydrological responses in a natural forested headwater before and after subsurface displacement

Abstract

Subsurface displacement can be a sign of shallow or deep-seated landslides. Although forested headwaters feature primarily active hydrological responses and erosional responses, the impacts of invisible subsurface displacement on hydrological responses are rarely reported. This study presents the changes in hydrological responses before and after subsurface displacement in a natural forested headwater. Using event-based analyses, the maximum values of rainfall, pore water pressure at the soil–bedrock interface, and surface runoff during rainfall events over 2 years were used to evaluate the changes in peak responses after subsurface displacement. The results showed that subsurface displacement increased the maximum pore water pressure, but reduced the rate of increase in pore water pressure, particularly for the measurement points where perennial subsurface saturation existed. Peak runoff was produced with at least two sources of near-surface flow and subsurface flow. Subsurface displacement reduced both the level and rate of increase in peak runoff. The internal relations in pore water pressure and the correlations between pore water pressure and runoff weakened after displacement. This weakening was likely due to displacement-induced cracks facilitating rainwater percolation into bedrock layers, and thereby decreasing the linkage between subsurface saturation and surface runoff. Subsurface displacement also modified the response time such that the maximum responses in pore water pressure and runoff tended to occur earlier than maximum rainfall during the period after displacement. This study demonstrates that invisible subsurface displacement directly changes hydrological behaviors, and that a headwater area is active, sensitive, and prone to disturbance, even if it is covered by forest.