[A-CG48] Water and sediment dynamics from land to coastal zones

[ACG48-P05] Long-term observations reveal runoff response during heavy rainfall in mountain catchment

*Yuko Asano¹, Kojyu Kishimoto¹, Hirokazu Haga² (1.Graduate School of Agricultural and Life Sciences, The University of Tokyo, 2.Tottori University graduate school agriculture study course)

Keywords:Rainfall-runoff data, granite, groundwater level, peak lag time, direct runoff

The runoff response of mountain streams during heavy rainfall is still unknown due to the difficulty of acquiring data. The Shirasaka experimental watershed (88.5 ha) of the Ecohydrology Research Institute has been observed for nearly 100 years and includes data from heavy rainfall events. It has been reported that in watersheds composed of accretionary sedimentary rocks, rainfall intensity and runoff intensity become almost equal when cumulative rainfall exceeds 100 to 200 mm, and we show that the same response occurs when rainfall exceeds 300 mm in granite basins based on observation data from experimental watersheds of Ecohydrology Research Institute. In order to elucidate what is happening in the catchment when runoff intensity is equal to rainfall intensity, we examined the historical records of the Shirasaka watershed and compiled data on groundwater levels at intervals of 1 minute to 1 hour from two wells located in the upper and lower position of the head hollow of a 1.18 ha North valley catchment, which is nested in the Shirasaka Watershed. During the heavy rainfall in June 1961 (total rainfall 567 mm), the delay of the peak runoff to the peak rainfall was almost zero minutes in the North valley and within 20 minutes in the Shirasaka watershed. In the Shirasaka watershed, the arrival time of the peak runoff was delayed by less than 20 minutes compared to the North valley, but the specific discharge was almost the same. During the period when rainfall intensity = runoff intensity, there was little change in groundwater levels along the valley, which rose to near the surface, and the water surface gradient was nearly constant, but the volume of runoff from the watershed increased or decreased more than twice as much, from 0.6 to 1.6 mm/5min, in response to changes in rainfall. The rapid runoff response was suggested to be mainly due to the propagation of hydraulic head.