We developed a prototype of the now-called “thermo-time domain reflectometry probe” in 1996. We revisited the article describing the prototype and explained how we thought at those moments. Let us start our visit; first met a soil water sensor at college. Later year at Texas A\&M University as a graduate student, Jim Heilman and Kevin McInnes became my supervisors. They inspired me to make a multipurpose time domain reflectometry (TDR) probe by combining TDR and dual-probe heat-pulse (DPHP) sensors. Our new TDR-DPHP sensor owed the hard work of Clerk Topp, Gaylon Campbell and Keith Bristow. When we published an article on the new TDR-DPHP sensor in 1996, Robert Horton of Iowa State University might be the only one who truly understood the potential of the new probe in the world. He and his research group named our sensor a “thermo-time domain reflectometry probe” and have been pushing its limit since then.

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Artificial macropore introduction and no-till practices were conducted in the subtropical red soil of a sugarcane field on Ishigakijima Island, Japan, to increase the storage of soil water and carbon. The examined fields were suffering from heavy rain, typical of the subtropical climate, the intensity of which is increasing because of climate change. Heavy rain causes surface runoff and soil erosion, which decreases sugarcane field. Soil particles were being delivered directly into the sea, potentially affecting coral reefs. Therefore, the storage of soil water and soil carbon is crucial for protecting the natural subtropical environment. Artificial macropores with bamboo fibers were installed at 1 m intervals to enhance vertical infiltration. In addition, no-tillage agriculture with the retention of plant residues was conducted to protect the surface soil from the impact of heavy rain. Field management trials were conducted under no-tillage with/without macropores (NT-M and NT-X, respectively) and conventional tillage with/without macropores (CT-M and CT-X, respectively). Rainfall, surface runoff, soil erosion, soil moisture, and total soil carbon were measured. As a result, the macropore installation fields (CT-M and NT-M) had greater soil moisture contents than the fields without macropore installation (CT-X and NT-X). The average soil moisture values of each profile revealed that NT-M retained more soil water than CT-X (p < 0.01). Moreover, the calculated soil profile total carbon decreased under NT-X, CT-M, and CT-X. This is normal for a tropical climate with a relatively high temperature. However, the NT-M treatment showed an increase in soil carbon by +0.45 t-C･ha−1･yr−1. This value corresponded well with that of conservative practices in such agriculture fields. This result indicates that macropore installation in combination with no-tillage practices increases the total carbon in the soil profile, even in tropical climates. We recommend macropore installation with no-tillage practices with plant residue retention as an effective measure for enhancing the retention of soil water and soil carbon.

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Since colloidal particles in soils are known to act as a carrier of contaminants, understanding the transport behavior of colloidal particles in the soil is an important issue. This study investigates the transport behavior of colloidal particles during the cation exchange reaction between Ca2+ and Na+ at the surface of porous media. One-dimensional column experiments were conducted using Toyoura sand as the repacked porous media and carboxyl latex particles as the model colloid, respectively. It was found that colloidal particles deposited on the surface of the Toyoura sand during the cation exchange reaction, regardless of the amount of exchanged cation and the particle size of the applied colloids. The deposited colloidal particles were released when the ionic strength of the pore water decreased after the cation exchange reaction. The interaction energies between the latex particles and the sand surface were calculated based on the DLVO theory, suggesting the contribution of the attraction in the secondary minimum to the colloid deposition during cation exchange reaction.

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A case study of measurements using Arduino was conducted to examine its applicability in evaluating the evapotranspiration (ET) from an upland field. A system to control an inexpensive CMOS sensor was assembled and used to measure air temperature and relative humidity. Assuming a light user, the basis of the system and its handling when using Arduino as a data logger were introduced. The minimal customization required to build a temperature-humidity measurement system was also described. The results show that the constructed system had a lower ET calculation accuracy than the conventional expensive and high-performance system, and that this was due not only to the sensor performance but also to the hardware customization. However, the analyses demonstrate that all operations of selecting and removing sensors with large individual specificity, setting appropriate conditions for removing undesirable data in the ET calculation method, averaging the data, or bias correction for individual specificity were effective at attaining acceptable accuracy as compared with that of the conventional system. The accumulation of information on such measurements is essential for promoting the utilization of a low-cost open-source system.

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