興津川流域におけるみかん園の分布と小気候的性格について

抄録

Predominance of mandarin orchards and tea gardens is a typical landscape on the Pacific coast of Shizuoka Prefecture. They are distributed not only on the climatically advantageous coastal strips but also some distance inland along river valleys where climatic conditions are less favourable. Mandarin having economical advantages over tea in recent years, there has been a tendency that mandarin orchards replace tea gardens, or even paddy fields in this region.
A distinct contrast in the distribution patterns of mandarin orchards and tea gardens can be observed along the River Okitsu within a relatively short distance. The valley, only 20 kilometers in length, can be divided by the presence of a narrow gorge in the halfway into the upper and the lower parts (Fig. 1). The lower part is characterized by predominance of mandarin orchards. They cover the greater parts of the sunny side of the mountain slopes and the valley floors as well, although the floor has only recently been turned into mandarin from paddy fields. Tea gardens are found only on the windswept ridges. On the contrary, the upper part of the valley is quite dissimilar in that mandarin orchards are concentrated to the height of 200400 meters above sea level and tea gardens are distributed both on the ridges and on the lower parts of the slopes (Fig. 2 and Photo 1). There seems to be no such tendency for the tea gardens in the valley bottom to be replaced by mandarin.
This areal difference in landscape is expected to be accounted for, at least partly, by the different local climatic conditions. Two of the students of Tokyo Metropolitan University, S. TAKABAYASHI and H. KITAJIMA, conducted some meteorological observations along the valley and reported the results in manuscript forms. The authors try to supplement their works and to make a further discussion in this paper.
The observations were made mostly during the nighttime in different seasons. Bimetal thermographs and mercury minimum thermometers were installed at different heights (ranging from 90 meters at the valley bottom to 400 meters at the top) of the slopes, and simultaneous temperature observations were carried out at some 20 localities with the help of high school pupils and university students. In addition, moving observations was conducted by using a thermister thermometer installed on a car.
Temperature inversion was commonly observed on clear nights in the colder seasons both in the upper and the lower parts of the valley, whereas it seems to be absent in summer. Particularly in winter the temperature difference between the valley bottom and the upper part of the slope (relative height being 250 meters) may reach 4.0°C or even greater (Fig. 3 and 4). The top of the inversion was not ascertained by the observations, but the ridge appear to be cooler irrespective of the altitudes.
There was no significant difference in the intensity of the inversion between the upper and the lower parts of the valley. Presence of temperature inversions itself does not explain the difference of the mandarin distribution. The explanation seems to lie in the fact that there is a remarkable difference in temperature. Many of the simultaneous observations revealed that there is a greater temperature gradient near the boundary between the upper and the lower parts of the valley (Fig. 5). In other words, the valley bottom of the lower part is much less frequently exposed to low temperatures. On the contrary, the upper valley is filled up with cold air, which is not easily drained out to the downstream. Consequently a typical cold air lake results, and the valley bottom there is subject to extremely low temperature.