11:00 AM - 1:00 PM
[AHW23-P05] Measurement of pore-air entrapment in soil layer in two small headwater catchments
Keywords:Rainfall-runoff processes, Pore-air pressure, Pore-air entrapment, Soil layer, Headwater catchment
Since the 1980’s, an important issue in the study of rainfall-runoff processes in forest catchments has been to understand how old water stored in the slope rapidly discharge during rainstorm event. To explain it, preferential runoff processes such as pipe flow, flow at the soil–bedrock interface, and subsurface flow in bedrock fractures have been focused. However, it is still difficult to predict the peak discharge during rainstorm.
Experimental studies have shown that entrapped pore-air in the unsaturated soil layer between wetting front and the underlying groundwater table will be compressed by the increase of infiltrating rainwater. After 2000’s, numerical simulations have showed that compression of entrapped pore-air due to infiltration cause increase in discharge. However, in the field measurement studies, 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. There have been few reports of pore-air behaviour in the field and the methods are not clear. Therefore, in order to examine the effect of compressed entrapped pore-air in the field, we need to develop the method to detect the pore-air pressure and confirm the formation of compressed entrapped pore-air in the field.
We observed the behaviour 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 as the positive pressure difference between Pair and Patm (ΔP = Pair – Patm). The Pair observation was conducted together with measurements of pore pressure (by tensiometer), throughfall, soil moisture and groundwater level in order to capture the feature of pore-air entrapment phenomenon. Observation was conducted in two small headwater catchments. One was a sub-catchment of Tsukuba Experimental Watershed (TC, area: 3.6 × 10-3 km2) and another was a sub-catchment of Hitachi-Ohta Experimental Watershed (HA, area: 8.4 × 10-3 km2), respectively. Both sites are located in Ibaraki Prefecture, Japan (Shimizu et al., 2021, Kubota et al., 2018). Those two catchments are different in soil depth and discharge characteristics, and similar in geology and rainfall pattern. Probes were installed along the valley line of the slope to focus on the lateral linkage of the pore-air in the soil layer and also the linkage to the runoff process. The soil depth along the measurement line in TC was 1–2 m, with a deep layer of more than 4 m in the middle, whereas in HA, the soil depth along the measurement line was either less than 1 m or in the range of 1–2 m. It is 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.
As a result, positive ΔP was observed corresponding to the rainfall peak during rainfall event at both catchments. The response of pore pressure and volumetric water content showed that the pressure rise of pore pressure was due to rise in pore-air pressure rather than pore-water increase. And therefore it is suggested that compression of entrapped pore-air was successfully observed with our instrument. The number of pore-air entrapment events detected were 21 at HA (thinner soil catchment), while it was 6 at TC (thicker soil catchment) from October 2018 to November 2019. Formation of pore-air entrapment was observed even in a weak rainfall of less than 10 mm/h. Duration times of pore-air entrapment were ranged from a few hours to a week. Relation between ΔP and rainfall-runoff process are suggested in the field and therefore accumulation of such data will enhance the discussion of rainfall-runoff processes in the future.
References
Shimizu et al., (2021) Hydrological Processes, 35: e14376. (DOI: 10.1002/hyp.14376)
Kubota et al., (2018) Bulletin of Forestry and Forest Products Research Institute, 17(1), 63-73.
Experimental studies have shown that entrapped pore-air in the unsaturated soil layer between wetting front and the underlying groundwater table will be compressed by the increase of infiltrating rainwater. After 2000’s, numerical simulations have showed that compression of entrapped pore-air due to infiltration cause increase in discharge. However, in the field measurement studies, 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. There have been few reports of pore-air behaviour in the field and the methods are not clear. Therefore, in order to examine the effect of compressed entrapped pore-air in the field, we need to develop the method to detect the pore-air pressure and confirm the formation of compressed entrapped pore-air in the field.
We observed the behaviour 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 as the positive pressure difference between Pair and Patm (ΔP = Pair – Patm). The Pair observation was conducted together with measurements of pore pressure (by tensiometer), throughfall, soil moisture and groundwater level in order to capture the feature of pore-air entrapment phenomenon. Observation was conducted in two small headwater catchments. One was a sub-catchment of Tsukuba Experimental Watershed (TC, area: 3.6 × 10-3 km2) and another was a sub-catchment of Hitachi-Ohta Experimental Watershed (HA, area: 8.4 × 10-3 km2), respectively. Both sites are located in Ibaraki Prefecture, Japan (Shimizu et al., 2021, Kubota et al., 2018). Those two catchments are different in soil depth and discharge characteristics, and similar in geology and rainfall pattern. Probes were installed along the valley line of the slope to focus on the lateral linkage of the pore-air in the soil layer and also the linkage to the runoff process. The soil depth along the measurement line in TC was 1–2 m, with a deep layer of more than 4 m in the middle, whereas in HA, the soil depth along the measurement line was either less than 1 m or in the range of 1–2 m. It is 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.
As a result, positive ΔP was observed corresponding to the rainfall peak during rainfall event at both catchments. The response of pore pressure and volumetric water content showed that the pressure rise of pore pressure was due to rise in pore-air pressure rather than pore-water increase. And therefore it is suggested that compression of entrapped pore-air was successfully observed with our instrument. The number of pore-air entrapment events detected were 21 at HA (thinner soil catchment), while it was 6 at TC (thicker soil catchment) from October 2018 to November 2019. Formation of pore-air entrapment was observed even in a weak rainfall of less than 10 mm/h. Duration times of pore-air entrapment were ranged from a few hours to a week. Relation between ΔP and rainfall-runoff process are suggested in the field and therefore accumulation of such data will enhance the discussion of rainfall-runoff processes in the future.
References
Shimizu et al., (2021) Hydrological Processes, 35: e14376. (DOI: 10.1002/hyp.14376)
Kubota et al., (2018) Bulletin of Forestry and Forest Products Research Institute, 17(1), 63-73.