写真と図で見る「37年ぶりの大渇水」

─小笠原諸島父島，母島における少雨時（2016～2017年）と平常時（2018年）の状況の比較─

抄録

 A period with less than average precipitation on Chichi-jima and Haha-jima in the Ogasawara (Bonin) Islands between 2016 to 2017 was compared to 2018 (normal year), based on field surveys conducted in March 2017 and February 2018. Water consumption showed a six-day cyclic variation, reflecting the voyage schedule of the marine vessel Ogasawara-maru connecting Chichi-jima and central Tokyo. Water consumption increased on the day Ogasawara-maru arrived at Chichi-jima, because the number of passengers sometimes corresponded to about 30% of number of residents on Chichi-jima. Compared to Chichi-jima, annual precipitation on Haha-jima was about 100 mm greater during the period 2007-2017, along with distinctly greater precipitation in May compared to April and June. In addition, precipitation on Haha-jima in August was the highest among all months. Even during a severe drought in 2016-2017, precipitation in August 2016 was greater than the average from 2007 to 2017 on Haha-jima, recovering water stored by the Chibusa dam which supplies drinking water on Haha-jima. The effective water storage per capita on Haha-jima is 1.5 times more than that on Chichi-jima, which in some part resulted in the temporal change in the percentage of water storage always being larger on Haha-jima. Both on Chichi-jima and Haha-jima, people tried to overcome the difficult situation of the severe drought, reflecting efforts by the local government to communicate the need to save water.