9:05 AM - 9:20 AM
[AHW20-02] Behavior of pore-air entrapment in unsaturated soil layer in two small headwater catchments with different soil depth
Keywords:Rainfall-runoff process, Pore-air pressure, Pore-air entrapment, Soil layer, Headwater catchment
Many researches have been conducted to clarify the mechanism of rainfall-runoff process in a forested catchment in humid warm-temperate regions, however it remains only partially understood. Since the 1980’s, the important issue in rainfall-runoff process studies has been solving how pre-event old water stored in the slope discharge in rapid response during rainstorm event. To explain it, there has been a focus on preferential flows, such as flow in macro-pore, pipe flow, flow in the soil–bedrock interface, and subsurface flow in bedrock fractures. One of other possible explanations of the rapid outflow of pre-event old water is the effect of pore-air entrapment. Experimental studies have shown that pore-air entrapment in the unsaturated zone of soil by infiltrating rainwater and the underlying groundwater table will effect on discharge. Although there have been few reports of pore-air behavior in the field, entrapped pore-air may have an impact on stream discharge.
Despite the fact that the pore pressure measured using a tensiometer is the sum of the pore-water pressure and the pore-air pressure (Pair), Pair has been neglected. We observed the behavior of Pair in the soil layer in a mountainous slope using a simple handmade probe together with the atmospheric pressure (Patm). Pore-air entrapment and its compression was considered to be detected by the positive pressure difference (ΔP = Pair – Patm). Also, we expected that entrapped pore-air develops more frequently in a thinner soil catchment because the space between the ground surface and the bedrock is smaller. Thus, two catchments with thin and thick soil were selected for this study: One was a sub-catchment of Hitachi-ohta Experimental Watershed (HA) and another was a sub-catchment of Tsukuba Experimental Watershed (TC), respectively. Both sites are located in Ibaraki Prefecture, Japan, and observations were conducted for about 1 year. As a result, the number of pore-air entrapment events detected were 21 at HA (thin soil catchment), while it was 6 at TC (thick soil catchment). The values of ΔP at the discharge peak were correlated with peak discharge at one of the nearby spring observation points in HA, suggesting the importance of the pore-air entrapment in the rainfall-runoff process, particularly for a headwater catchment with relatively thin soil.
Despite the fact that the pore pressure measured using a tensiometer is the sum of the pore-water pressure and the pore-air pressure (Pair), Pair has been neglected. We observed the behavior of Pair in the soil layer in a mountainous slope using a simple handmade probe together with the atmospheric pressure (Patm). Pore-air entrapment and its compression was considered to be detected by the positive pressure difference (ΔP = Pair – Patm). Also, we expected that entrapped pore-air develops more frequently in a thinner soil catchment because the space between the ground surface and the bedrock is smaller. Thus, two catchments with thin and thick soil were selected for this study: One was a sub-catchment of Hitachi-ohta Experimental Watershed (HA) and another was a sub-catchment of Tsukuba Experimental Watershed (TC), respectively. Both sites are located in Ibaraki Prefecture, Japan, and observations were conducted for about 1 year. As a result, the number of pore-air entrapment events detected were 21 at HA (thin soil catchment), while it was 6 at TC (thick soil catchment). The values of ΔP at the discharge peak were correlated with peak discharge at one of the nearby spring observation points in HA, suggesting the importance of the pore-air entrapment in the rainfall-runoff process, particularly for a headwater catchment with relatively thin soil.