A New Method for Determining the Dangerous Area for Cold Damage of Citrus Trees

Abstract

In warm districts of Japan, we have a great many of citrus plantations. Recently, many planters in these districts are desirous to enlarge their orchards to increase their incomes.The local climate, however, is markedly various in each location even in a small area of just one orchard. Especially in winter the cold air subsides to the lower parts of the slope land, making such places very dangerous for citrus plantation because of cold damage. Therefore, when planters plan to make new citrus orchards, it is very important to choose suitable locations, especially to omit, in their planed location, all dangerous spots where cold damage may occur. Therefore, it will become an important problem for the planters to find such dangerous spots before planting citrus trees.The author offers a new method of determining dangerous spots for new plantations. The method is based on the following four factors:(1) return period at the nearest meteorological observatory, (2) the relationship of the low air temperatures between the investigating area in question and the observatory, (3) the critical minimum temperature limit, and(4) the distribution of low temperatures in the location.1. The return period of the minimum temperature in winter, which will happen once every 10 years, can be got from the statistics of the past 30 or more years at the observatory near the investigating location.2. The relationship of low temperatures between the observatory and the base station newly set up in the location may be obtained from the regression formula. This formula is made up from the minimum temperatures registered at the two stations on clear, calm nights during which the most cold damage usually occurs.In order to carry out this investigation, the minimum temperatures at the new base station built on the investigating location, have to be observed every day during the winter.3. The critical minimum temperature causing the cold damage will be decided through investigating both physiological and economical conditions in each prefecture in warm districts. The critical minimum temperature from a physiological point of view is shown to be -7°C, from the past records of investigations and experimental researches.4. The distribution of the minimum temperatures in the investigating location has to be got from the result of new observations on several clear and calm nights.To make clear this distribution, many new observation points must be set up around the entire area of the location. From these results the planters will be able to show some spots apt to be relatively colder than other spots on clear and calm nights in winter.Through the discussion of these results, some dangerous spots in the lacation may be pointed out.The author emphasizes, furthermore, that a more important fact is the pointing out of practical measures to improve microclimatological conditions at such dangerous spots, which make them safer by, for example, arranging some belts of hedges or woods on upper parts of the slope to change the cold air flow, or cutting them off on the lower parts to exhaust cold air accumulated there.It is very important, therefore, not only to point out the dangerous spots in the investigating location, but also to give practical ways and means for clearing away a so-called cold air pond to explain. how and why such cold air ponds are created.