Climate change impacts
Okinawa Prefecture is an island prefecture composed of 160 coral reef islands. As there is a dearth of significant water sources such as large rivers and lakes, and precipitation varies greatly depending on the year and season, the stability of the water supply is not optimal. More remote islands in Okinawa Prefecture face particular challenges in this respect, having to implement water-use restrictions in the event of drought and the securing of emergency water supplies in the case of natural disasters or other major emergencies. According to the "Okinawa Long-Term Water Supply and Demand Plan 2019" published in April 2019, it has been necessary to implement water restrictions regularly, especially in the Zamami locale, and it is a matter of concern that climate change may subject water resources to increasingly severe drought conditions in this area.
Adaptation activity
To address these issues, with a 2016 Okinawa Promotion and Development Special Subsidy, Okinawa Prefecture installed two portable reverse osmosis seawater desalination units (Fig. 2) which operate by using a pressure differential to cause salinized feedwater to pass through a membrane thereby separating the salt and product water (Fig. 1). The capacity of each unit is 200 m3/day, which is enough to supply about 220 households (about 570 people).
In 2017, the monthly precipitation rate in Okinawa Prefecture in August was only 19% of the average year, the lowest ever recorded. After the rainy season, the number of dry days has continued, and as a result, Utaha reservoir, the main water supply of Aka Island in Zamami-mura, was under severe strain, hence the decision was made to provide Zamami-mura with temporary use of a portable seawater desalination unit for the first time since it had been acquired by Okinawa Prefecture.
A unit was also installed in Minamidaitō-mura after a breakdown of the regular water purification plant (seawater desalination plant). Thanks to a rapid response by the local authority and contractors, the unit was transported and put into service in just three days (Fig. 3).
Outputs / Expected benefits
The introduction of portable seawater desalination units will enable Okinawa Prefecture to establish an effective back-up system in the event of natural disasters, emergencies, droughts, etc., and to ensure the stability of water supplies.
Fig. 3 Portable Seawater Desalination Unit being installed in Minamidaitō
(Source: Okinawa Prefectural Enterprise Bureau website, "Major News of 2020, Enterprise Bureau")
