散水氷結法による防霜対策に関する研究

(3) 有効散水強度の下限値

抄録

In the western parts of Japan, there are frequent risks of frost damage on several crops such as tea plants, vine and others in the late spring. Experimental studies on protection of plants from the cold by sprinkling method utilizing latent heat of solidification were carried out to make establish practical application methods for the sake of saving water consumption.
The main purpose of the experiments is to find the lowest value of effective sprinkling intensity under several air temperature conditions. Through discussions on the fitness of spray catching ratio, the experimental results were examined with the theoretical values by Niemann's and by the author's.
Some results found in the experiments at cold night with radiative cooling are summarized as follows:
1) Temperature of iced leaf under sprinkling could not increase up to 0°C under both conditions of lower air temperatures and smaller sprinkling intensity. There were clearly critical point of sprinkling intensity, which could maintain leaf temperature around 0°C. These points which are defined as the lowest value of effective sprinkling intensity were also changed with surrounding air temperatures. They became larger as air temperatures became lower. The relations between them was a linear one (Fig. 3.). Having compared the measured values with the theoretical values in their tendencies in the graph, it was found that the Niemann's one differed from the actual one especially in the lower temperature range but the author's one agree well throughout the giving range of temperature.
2) The spray catching ratio measured using grouped branches with leaves of cranberry tree (Viburnum awabuki K. Koch) showed almost constant values in the range from 0.1 to 4.6mm·hr^-1 of sprinkling intensity and was about 0.29 on average (Fig. 4.). The theoretical values by Niemann's method changed from 0.27 (at O°C) to 0.36 (at -4°C). On the other hand, the author's one was 0.27 (0--4°C), which was more close value to the experimental one.
3) The fall-off of leaf temperature after 20 minutes of sprinkling-off changed proportionally to the initial leaf temperature, when air temperatures were -3.0--5.8°C. They became larger as initial leaf temperature became higher (nearly equal to 0°C). These relations were also changed parallely with air temperatures, that is, the fall-off of leaves temperatures became large as lowering of air temperatures.
4) After re-sprinkling, the iced leaf temperature which was lowered to around air temperature began to increase showing the shape of saturation curve. The larger the sprinkling intensity was, the shorter the required time to make plant body temperature 0°C was. But in the case of smaller intensity of sprinkling, the required time became longer exponentially.