Journal of the Japanese Society of Soil Physics

When installing TDR probes to rock, void spaces (gaps) between the probe rods and rock may be en-countered. In this study, the dielectric constants of water and air were measured with various controlled gaps and ef-fects of the gaps were experimentally investigated. We fo-cused on a two-rod probe for which analytical and numer-ical solutions were available. Symmetric longitudinal gaps (identical uniform gaps along both rods), non-symmetric longitudinal gaps (uniform gaps along only one rod), and end gaps beyond the end of the rods were considered. For the symmetric cases, measured dielectric constant values of water and air were compared with analytical and/or nu-merically simulated values, and a close agreement was ob-served. For the non-symmetric cases, such agreement was seen only when the gap width was relatively small. As the gap widths increased, the dielectric constant of wa-ter was underestimated compared to the numerically esti-mated values. Such underestimation was more significant when the gaps were along the rod connected to the center conductor, suggesting that the sensitivities of the two rods were uneven. The effect of the end gaps was very lim-ited whereas the longitudinal gaps, e.g., due to loose guide holes or soil shrinkage can lead to significant measurement errors.

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Frozen soil layer sometimes impedes the snowmelt infiltration into deep soil layer, which decreases the amount of soil water recharge and delay the transporta-tion of nitric acid to deep soil layer. To investigate the rela-tionship between soil frost and snowmelt infiltration, three years field observation was conducted at the arable fieldlocated in Tokachi region in northernmost island of Japan. Two plots were prepared and snow on the one plot was re-moved for approximately one month to enhance the soil freezing. As a result, soil frost depths at the beginning of snowmelt period were ranged between 0.1 and 0.5 m. The amount of infiltrated water into below 0.5 m depth during snowmelt period was calculated from the field data. The strong relationship between the frost depth and the ratio of snowmelt infiltration to the available snowmelt water (in-filtration ratio) was obtained, whereas there were no clear relationships between the infiltration ratio and soil temper-ature at 0.05 m depth just before the beginning of snowmelt period. Thus, the frost depth is one of the most important factors to estimate the amount of snowmelt infiltration into deep soil layer when the frost depth is relatively shallow. We explained the process of the decrease in the snowmelt infiltration with the increase in the soil frost depth using a conceptual model.

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