Flow paths, rainfall properties, and antecedent soil moisture controlling lags to peak discharge in a granitic unchanneled catchment

[1] The lagtime between peak rainfall and peak discharge is an important index reflecting hydrological properties in a catchment. To characterize lag times, we studied the effects of rainfall properties, antecedent moisture conditions, and flow paths on runoff response in a forested unchanneled catchment, using a hydrometric and hydrochemical approach. Soil moisture, soil pore water pressure, and piezometric surface were monitored. Also, dissolved silica and organic carbon concentrations of spring water and subsurface water were observed. Runoff response was characterized by two types of lag times: a short lag time (< 2 hours) or a long lag time (> 24 hours). During events with short lag times, saturation excess overland flow was dominant, and the source area was limited to the near-spring area. During events with long lag times, saturated subsurface flow above the soil-bedrock interface was dominant, and the source area near the spring was connected to the upslope area via saturated zone above soil-bedrock interface. The spring-hillslope hydrological connection to generate peak discharges with long lag times occurred when the sum of cumulative rainfall and an antecedent soil moisture index, which was derived from initial storage of surface soil layer, was greater than 135 mm. Moreover, the time between the upslope connection of source area and subsequent peak discharge decreased with the average rainfall intensity in the time. We conclude that consideration of antecedent soil moisture conditions as well as rainfall amount and intensity is essential for understanding the regional characteristics of lag times and subsurface water movement.