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Unsaturated soil contains pore air, some of which becomes entrapped in the soil layer during the infiltration process. Such entrapped pore air in the soil layer inhibits infiltration, decreases hydraulic conductivity, and causes erroneous estimates of groundwater response. Experimental studies suggest an effect of compressed entrapped pore air on stream discharge. However, few studies have investigated the behaviour of pore air in the field, and no method for measurement of pore‐air behaviour has been established. We measured pore‐air pressure (Pair) with a simple handmade probe, combined with atmospheric pressure (Patm) and groundwater level. The entrapment and compression of pore air were detected based on the pressure difference (ΔP = Pair – Patm). Observations were conducted for approximately 1 year in two small headwater catchments (TC and HA, which have differences in soil depth and groundwater level dynamics) in Ibaraki Prefecture, Japan. Positive responses of ΔP during some rainfall events were confirmed at both TC and HA. Pore‐air entrapment occurred even during weak rainfall events with rainfall intensity of <10 mm/h. During a typhoon event in October 2019, a maximum ΔP of 3.9 kPa was observed at HA. The pore pressure increase recorded by the tensiometer was mostly explained by the increase in ΔP, which was supported by the soil moisture data. Therefore, we concluded that our handmade measurement system could successfully measure Pair and detect pore‐air entrapment in the field. This showed that Pair is not negligible. Total event rainfall was related to the maximum ΔP. It also showed that the rise in the groundwater level plays a significant role in pressurization of the entrapped pore air. We present the first field data demonstrating the effect of groundwater level rise on the pressurization of entrapped Pair. [ABSTRACT FROM AUTHOR]

Inter‐catchment groundwater flow (IGF) plays an essential role in streamflow generation and water quality in forested headwaters. Multiple factors are thought to contribute to IGF, including climate, topographical, and geological factors. However, studies have not clarified the relationships between IGF and catchment properties in the headwater catchments due to the lack of observational data at scales smaller than 100 ha. This study examined possible factors influencing IGF using random forest analysis based on annual water balance data from 152 forested catchments ranging from 0.09 to 9400 ha in Japan. The results showed that catchment scale had the greatest influence on IGF, and IGF tended to decrease with increasing catchment area at scales of less than 10 ha. The average IGF stabilized around zero in catchments greater than 10 ha. The averaged IGF trend with catchment scale indicated more outward groundwater flow in catchments smaller than 10 ha, but no relationship between IGF and catchment size in catchments larger than 10 ha. The variability in IGF decreased with catchment size and was lowest at 10–100 ha. The decrease in variability in catchments less than 100 ha was mainly due to river confluence and the increased variability in catchments larger than 100 ha indicated potential observation errors increase in catchments of this size. Plain Language Summary: Inter‐catchment groundwater flow (IGF) refers to groundwater flux across surface topographic boundaries. Recent studies have clarified that IGF significantly affects streamflow generation and water quality. However, direct observation of IGF is difficult, and the mechanisms underlying IGF are not fully understood. This study examined the factors influencing IGF, as well as its scale dependence and variability based on water balance data from 152 forested catchments ranging from 0.09 to 9400 ha in Japan. This showed that catchment area had the greatest influence on IGF. Groundwater tended to flow out of catchments smaller than 10 ha, and stabilized in those larger than 10 ha. Variability in the IGF decreased with catchment size at less than 100 ha. These results suggest that on the scale of hillslope and headwater catchments, IGF has a great influence on streamflow generation. Key Points: We examined the factors influencing inter‐catchment groundwater flow (IGF) using water balance data for 152 forest catchments in JapanCatchment scale was the factor with the greatest influence on the IGFGroundwater tends to flow out of catchments smaller than 10 ha and IGF approaches zero when the catchment area exceeds 10 ha [ABSTRACT FROM AUTHOR]

The size of stemflow infiltration areas around the boles of trees is currently a topic of interest and debate within the hydrologic community. There is a gap in our knowledge of stemflow infiltration areas in many wooded ecosystems and a need for more than the few studies that have examined stemflow infiltration areas directly. Hence, this field study was specifically undertaken to mitigate the existing data gap by providing direct measurements of stemflow infiltration areas from high stemflow‐producing American beech (Fagus grandifolia Ehrh.) trees. Different stemflow rates (290, 72 and 31 L h−1) were simulated using dye‐infused stemflow and the areas of stemflow infiltration around four trees determined by measuring the areal extent of dye on the soil surface. Our results revealed that stemflow infiltration areas ranged from 0.0035 to 0.0951 m2 tree−1. The mean basal area funnelling ratio was 46.5 ± 1.8, whereas the funnelling ratios per unit infiltration areas, FP,It, were between 32.0 and 258.4. Despite intentionally high stemflow rates, chosen to compensate for the high infiltration capacities of forest soils, these results reinforce the fact that stemflow is an extremely localized input in natural forests. Thus, these results, even if specific to F. grandifolia within a particular forest and soil type, support a growing body of work indicating that stemflow infiltration areas are usually <1 m2, and often much smaller, in natural forests. Moreover, the high values of FP,It provide further evidence indicating that stemflow inputs are important for the development of hot spots in near‐trunk soils. [ABSTRACT FROM AUTHOR]