Hydroclimatic conditions during the first half of the 20th century in the bonin (ogasawara) islands

概要

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According to past studies based on long-term variations in precipitation and air temperature, climate in Chichi-jima of the Bonin (Ogasawara) Islands was rather wetter during the first half of the 20th century than during the latter half through present. The present study revealed actual hydroclimatic conditions using detailed meteorological components on monthly basis in the Annual Report of the Central Meteorological Observatory in terms of estimating potential evaporation and wetness index. As results of reevaluation of potential evaporation, Thornthwaite method underestimates potential evaporation especially during the cold season (from December to April) due to lower air temperature relatively to solar radiation. Thus, frequency of semi-dry month increased to 40-50% in this season. Annual wetness index is 1.24 in average (1907-1939) and only 5 years with less than 1.0 (semi-dry year). Small pan evaporation observed from 1931 to 1939 in Japan exhibits that Chichi-jima was one of the largest evaporation sites, namely the driest site in Japan although hydroclimatic conditions was wetter during the period.
首都大学東京の論文