SOIL TEMPERATURE OBSERVATIONS IN WESTERN SUBURBS OF TOKYO AND THEIR STATISTICAL STUDIES (2ND REPORT): EFFECT OF SOIL MOISTURE ON THE DISTRIBUTION OF SOIL TEMPERATURE

Abstract

In the previous report, the author discussed the effect of precipitation on the vertical distribution of soil temperature basing on his own observation. Since a part of the precipitation nourishes the soil moisture under the ground, it is probable that the moisture has an influence on the horizontal temperature distribution of the soil. In order to verify this fact, the author and his collaborators observed the distribution of the soil temperature at 75 cm depth using the method of moving observation. Obsevations were carried out in and around Hoya, the western suburbs of Tokyo (Fig. 1 and 2). The instruments used were thermistor thermometers for soil temperature, boringbars or -sticks, and bicycles for moving (Photo. 1 and 2). Reading of temperature was taken about 10 minutes after putting of the thermometers into the hole at 75 cm depth. Observation were made at about 80 points, favoured with the following three conditions ; bare ground, comparatively soft soil, open and sunny site. Errors of temperature readings due to the time lag caused by the moving were nearly negligible, because soil temperature at 75 cm depth shows neither daily nor interdiurnal change for a few days as reported in the previous paper. In winter, however, the interdiurnal change reaching about -0.1°C was found, so that the author made temperature adjustments by adopting a cross-check method. The meteorological conditions before, during and after the observations are shown in Fig. 3 and 6. The results obtained were summarized as follows In summer (see Fig. 5), 1. Colder areas where soil temperature is lower than 20°C exist along valleys floor with its surroudings, cultivated ground, and forests. 2. Warmer areas (higher than 21°C) are found in urbanized areas and some parts along the riverside. In winter (see Fig. 7), 3. Colder areas (lower than 11°C) are confirmed in urbanized areas and in some parts of the valley. 4. Warmer areas (higher than 12°C) exist in some parts with smaller house-density, cultivated upland and forests. Based on these facts, it may be concluded that the local horizontal distribution of soil temperature has seasonal changes, the regional difference of soil temperature is comparatively larger in summer and smaller than that of the air temperature in all seasons. The distribution pattern is relatively complicated in winter. This shows that the local distribution of soil temperature is more modified by many other factors than the solar radiation, such as river water, underground water, both of which were observed in autumn (Fig. 8). In this paper, the direct effect of the underground water on the soil temperature was not clarified, but the influence of the river water was found to some extent as shown by Fig. 10 and 11. The former was derived from above-mentioned observations of soil temperature distribution in July (T7) and December (T12), 1964. The relationship between temperature difference (T7-T12) and the distance L to the bank from the observation site are shown there. If the temperature difference is explained by the heat flux in the soil and the distance L represents soil moisture, the influence of the moisture on the annual heat exchange in soil can be shown by Fig. 10. In Fig. 11 we can find the correlation between the soil temperature and moisture which were observed in June, 1966. All of the above statements are provisional and nothing but a qualitative conclusion. So we have many important problems to be investigated. In particular, the relationship between the soil temperature and moisture should be studied from the view point of heat and water balance in the soil.