11:00 AM - 1:00 PM
[AHW23-P06] Discharge of precipitation component from hillslopes at the saturated riparian zone in a forested headwater
Keywords:precipitation, rainfall-runoff process, forested hillslope, riparian zone
Understanding discharge processes from forested hillslopes has been developed by focusing on the old water stored before the rain generating runoffs. In recent years, studies analyzing the transit time distribution of discharge water have shown the precipitation component makes a non-negligible contribution to runoff during rainstorms. It implies the need to re-evaluate the discharge process of the precipitation component from hillslopes. Therefore, the objective of this study was to clarify the runoff process during rainstorms in the saturated riparian zone, where precipitation falls directly and connects the hillslope to the stream, by focusing on the precipitation components.
Hydrological observations were undertaken from August 2020 to July 2021 in a forested headwater catchment. Water samples (rainwater, stream water, groundwater, soil water, and saturated surface water) were taken in periodic field surveys and the intensive field survey during a rainstorm and analyzed in oxygen and hydrogen stable isotopic compositions and inorganic anion concentrations.
An end-member mixing analysis using anion tracers was applied to the saturated surface water during the rainstorm, showing that the precipitation component contribution varied from 13.2% to 47.4%. Particularly, the contribution of precipitation components to the saturated surface water became larger when the rising speed of the hydraulic head in the riparian groundwater decreased. In addition, a subsurface layer above the groundwater table at the hillslope was saturated. Although the saturated subsurface area tended to expand in the downward direction, it always existed above the groundwater table level. Furthermore, there was a significant positive correlation between the hydraulic potential of the saturated subsurface area and the contribution of precipitation components to the saturated surface water. These results suggest that direct precipitation to the saturated surface area and saturated lateral subsurface flow from the hillslope are involved in the runoff of precipitation components during rainstorms. There has been the unified view that the direct precipitation discharge hardly occurs on the forested hillslope where the infiltration capacity is high. However, this study implied that the saturated surface area connecting the hillslope and the stream has the function of discharging precipitation components to the stream during rainstorms.
Hydrological observations were undertaken from August 2020 to July 2021 in a forested headwater catchment. Water samples (rainwater, stream water, groundwater, soil water, and saturated surface water) were taken in periodic field surveys and the intensive field survey during a rainstorm and analyzed in oxygen and hydrogen stable isotopic compositions and inorganic anion concentrations.
An end-member mixing analysis using anion tracers was applied to the saturated surface water during the rainstorm, showing that the precipitation component contribution varied from 13.2% to 47.4%. Particularly, the contribution of precipitation components to the saturated surface water became larger when the rising speed of the hydraulic head in the riparian groundwater decreased. In addition, a subsurface layer above the groundwater table at the hillslope was saturated. Although the saturated subsurface area tended to expand in the downward direction, it always existed above the groundwater table level. Furthermore, there was a significant positive correlation between the hydraulic potential of the saturated subsurface area and the contribution of precipitation components to the saturated surface water. These results suggest that direct precipitation to the saturated surface area and saturated lateral subsurface flow from the hillslope are involved in the runoff of precipitation components during rainstorms. There has been the unified view that the direct precipitation discharge hardly occurs on the forested hillslope where the infiltration capacity is high. However, this study implied that the saturated surface area connecting the hillslope and the stream has the function of discharging precipitation components to the stream during rainstorms.