This study evaluated the impact of the rapid change in unsaturated hydraulic conductivity near saturation, as described by the van Genuchten (VG) model, on the results of soil water movement simulations. Free drainage simulations from near-saturated conditions were conducted using HYDRUS-1D for four soil types, including soils containing a large amount of macropores, resulting in a small n value of the VG model parameter and large saturated hydraulic conductivity. The results showed that, for soils having a small n value, the VG model significantly reduced unsaturated hydraulic conductivity near saturation. In contrast, the modified VG model with the conventional air-entry pressure setting (−2 cm) suppressed this reduction. However, comparisons with observed data for soil containing macropores and having a small n value showed that the conventional modified VG model overestimated drainage rates. Conversely, setting he close to 0 cm allowed the model to reproduce the observed data well. A sensitivity analysis revealed that the effect of pore connectivity parameter, l is much less than that of the air-entry pressure, he in modified VG model. Because remarkable improvement on the calculation result was not obtained in spite of the change in l in the VG model, our results indicates the usefulness of modified VG model if we can determine he appropriately.

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Global Navigation Satellite System Reflectometry (GNSS-R), which utilizes satellite signals from GNSS orbiting the Earth for positioning, has attracted attention as a method for monitoring environmental parameters. GNSS-R is considered capable of estimating ocean surface wind speed, wave height, sea level and ice sheets, as well as soil moisture, vegetation, and snow depth on land, by analyzing the signals reflected off the Earth’s surface, among the satellite signals (electromagnetic waves) transmitted by GNSS. Receivers and platforms available for GNSS-R cover a wide variety of types, including not only ground stations but also ships at sea, airborne platforms such as balloons, aircraft, and UAVs (unmanned aerial vehicles), as well as satellites in orbit. The first part of this paper presented an overview of GNSS-R, covering available platforms, antenna mounting configurations, analysis techniques and its key characteristics. In the latter part of this paper, we focused on GNSS Interferometric Reflectometry (GNSS-IR), a specific form of GNSS-R that has seen remarkable research progress in recent years, particularly for estimating soil moisture using this technique. GNSS-IR is a method that uses the signal-to-noise ratio (SNR) as the analytical parameter and is highly scalable due to its applicability to GNSS base stations used for standard static positioning. In this paper, we outlined the principles and features of GNSS-IR, discussed its temporal and spatial resolution in measurements, the analysis methods, software used and the accuracy of soil moisture estimation. Finally, we discussed the current challenges and future prospects of this technique.

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GNSS Interferometric Reflectometry (GNSSIR) is a technique that estimates areal surface soil moisture at the field scale by analyzing interference between direct and ground-reflected GNSS signals received by near-surface antennas. In recent years, low-cost GNSS antennas and receivers have become increasingly widespread, and their application to GNSS-IR has been desired. However, low-cost receiving systems face challenges such as low data accuracy and limitations in the available satellites and frequency bands. In this study, we developed a low-cost GNSS-IR system using an L1-band GPS antenna, receiver, and Raspberry Pi microcomputer, with a total cost of approximately ¥ 10,000. Field observations were conducted using this low-cost receiving system to evaluate the accuracy of soil moisture estimation and the observation coverage provided by the tracked satellites. As a result, the calculated Fresnel zones of GPS signals captured by the system covered most of the field area, excluding approximately 60◦ in the northern direction where satellite flyovers were absent. From the Signal-to-Noise Ratio (SNR) interference patterns, we extracted three variables, phase offset (φo), signal amplitude (As), and effective reflector depth (Hdep) ̶ and compared them with in-situ volumetric water content (θm) obtained using dielectric probes. The correlation coefficients between θm and φo, As, and Hdep were r = 0.79,−0.43, and −0.63, respectively. Restricting the analysis to signals from azimuths of 120 – 240◦, where the influence of nearby buildings was expected to be smaller, the correlations improved to r = 0.78 (φo), −0.75 (As), and −0.82 (Hdep). Soil moisture estimates based on φo yielded an RMSE of 0.03 m3 m−3. These results suggest that surface soil moisture can be estimated using this low-cost GNSS-IR system, despite limitations such as the restriction to single-band L1 signals and NMEA format output. Limiting the analysis to unobstructed azimuths improved the correlations, suggesting that careful selection of satellite signal directions can improve retrieval performance.

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Accurate field measurement of evaporation requires weighing lysimeters that reproduce realistic hydraulic boundary conditions while operating reliably with minimal maintenance. This is particularly challenging in coastal reclaimed fields, where shallow and rapidly fluctuating groundwater and desalinization practices can drive the soil profile toward near saturation, limiting the performance of conventional equilibrium tension lysimeters that rely on vacuum regulation. We developed a Lower-Boundary-ControlledWeighing Lysimeter (LBC-WL) that regulates the lower boundary through bidirectional water exchange using a bidirectional tubing (peristaltic) pumping system and operates autonomously with a compact solar panel power supply. The system consists of a weighing inner cylinder whose base contains porous ceramic cups embedded in a silica sand layer and connected to an external water tank. A datalogger continuously compares the bottom pressure head inside the lysimeter (hb) with an external reference head measured outside (hout) and actuates the pump to maintain hb within a prescribed tolerance band around hout, with a short waiting time after each pumping action to improve stability. Laboratory single-step and multi-step tests demonstrated that the pumping system maintained hb close to prescribed targets over both relatively dry and near-saturated ranges while conserving mass balance between the lysimeter and the tank, consistent with the water retention behavior of the silica sand. Greenhouse tests confirmed continuous operation under solar panel power and showed that widening the tolerance band under near-saturated conditions reduced excessive pump cycling. Field deployment during desalinization in a coastal reclaimed area maintained lower boundary control (typically hb = hout±10 to ±15 cm, depending on wetness) and enabled evaporation to be quantified at 10-min resolution during the subsequent drying period. These results indicate that the LBC-WL provides a robust platform for studying evaporation and water balance processes in managed fields with shallow groundwater.

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