Understanding water behavior in porous media in microgravity conditions is essential for growing soil-cultivated crops in outer space. A drop tower is a cost-effective method to generate microgravity environments. Still, it provides only a few seconds or less microgravity, which is insufficient for observing time-consuming phenomena such as infiltration in soil. In this study, we proposed a new infiltration procedure, where infiltration occurred repeatedly for 0.4 s μG conditions generated by a 2 m high drop tower. The infiltration for the next 0.4 s μG proceeded after a new wetting front location was initiated by manually advancing the wetting front. Repeating this procedure, referred to the intermittent infiltration procedure hereafter, enabled us to observe long-term infiltration under microgravity condition. We measured infiltration rates in glass beads of 0.4 mm and 2 mm diameter using the intermittent infiltration procedure. The infiltration rate for 0.4 mm glass beads agreed with that observed in the continuous infiltration experiment, although that for 2 mm glass beads was underestimated. We anticipated that this limitation could be overcome by utilizing higher drop towers capable of generating several seconds of microgravity. The intermittent infiltration procedure opened affordable measurements of soil physical parameters under microgravity to address challenges associated with repetitive experiments. Numerical simulations were also conducted to evaluate the intermittent infiltration, assuming porous media as a bundle of capillaries, by comparing infiltration rates between continuous infiltration over an extended period and intermittent infiltration by integrating 0.4-second infiltration. For capillaries with a 0.3 mm inner diameter, the infiltration rates for intermittent and continuous infiltration remained consistent. However, the inertial regime must be considered for capillaries with an inner diameter greater than 1.5 mm, leading to underestimating infiltration rates.

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The purpose of this paper was to examine the changes in suspended particle size distribution with depth and time during filtration using double filter layer composed of a large artificial semi-cylindrical material in the upper and small quartz sand in the lower. As influent, clay suspension was adjusted to the maximum particle diameter of 70 and 10 μm and the concentration of about 4,600 mg L−1. At elapsed time 60, 120 and 240 min, the amount of the trapped particles in the upper layer, in the boundary between the upper and the lower layer, and in the lower layer was measured. Particle size distributions of the inflow and the outflow suspension, and the trapped particles mentioned above were also measured using laser diffraction scattering particle size distribution analyzer. The results showed the following; (i) in the case of the maximum particle diameter of 70 μm, the main capture site at elapsed time 60 min was the upper layer, but it shifted to the boundary at elapsed time 240 min. On the other hand, in the case of the maximum particle diameter of 10 μm, the capture sites of the sediment were both the upper layer and the boundary at elapsed time 60, 120 and 240 min, (ii) in all cases, the particle size distribution of the suspended particle reaching the lower layer was finer enough to enter the lower layer due to the trapping of the particles in the upper layer and the boundary, (iii) as the coarser part of the suspended particle was trapped in the lower layer, the particle size distributions of outflow suspension were similar to each other, independent f the maximum particle diameter and the elapsed time.

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To assess the potential of the COsmic-ray Soil Moisture Observation System (COSMOS) for measuring soil moisture on the slope of volcanic ash soil, we analyzed the response of its epithermal neutron counts (N) to the volumetric water content (θ ) recorded by soil moisture sensors installed at five locations and three depths. By comparing θ across these locations and depths, we estimated the maximum effective observation depth of COSMOS is 17 cm. Based on this estimate, we constructed four datasets comprising the mean θ values from TDT sensors at a depth of 10 cm (θAvg) and the corresponding N values, along with their 3-hour, 12-hour, and 24-hour averages. In all combinations, the correlation between N and θAvg was low, suggesting that the presence or absence of vegetation cover significantly influences N. Consequently, we classified the data into periods with and without vegetation cover — determined using camera imagery — and applied distinct calibration equations for each period. This approach significantly improved the agreement between COSMOSestimated θ and θAvg. Our findings demonstrate the effectiveness of COSMOS for monitoring surface soil moisture on volcanic ash slopes characterized by exceptionally high porosity of volcanic ash soil.

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