The water supply in Miyakojima involves the history and development of the current water supply on Miyakojima, a small coral island with only one river, which is administered by Okinawa Prefecture of southwestern Japan. Overview The water supply in Miyakojima depends predominantly on spring water or ground water, since there are almost no rivers. Although the yearly rainfall amounts to , 40% of it evaporates, 10% flows into the ground and 50% penetrates deep into the ground. Thus, there is almost no source of surface water. Geology Beneath the soil of Miyakojima is a Ryukyu limestone layer which is porous, highly permeable, and does not preserve rainwater. Under the limestone layer is a Shimajiri mudstone layer of bedrock, which is an impermeable rock. In the local dialect, it is called kucha. Tectonic movements accompanied by several faults have formed underground valleys, and groundwater flows along these valleys. The underground basins are divided into 22 valleys. Thus, 40 per cent of the abundant rainfall on Miyakojima penetrates the surface and quickly flows underground out into the ocean, unused. In addition, the strong subtropical sunshine evaporates as much as 50 per cent of the rainfall. The result is that only 10 per cent of the precipitation flows along the surface. Using natural water resources on Miyakojima has been quite difficult, owing to these natural conditions. List of ground water basins List of water supply sources History In the past, water in deep caves was used, as suggested by their language. Namely, the spring is called urigā, which means "going down into the cave and drawing water from the spring". It was usually several meters deep into the ground, in some cases, 20 meters by stepping down the stone steps. In the 13th century, excavation of wells started and in an early 18th-century document, 59 wells were recorded. Among them, Yamatsugā which was developed in 1720, was designated as an important site by the Hirara City (now Miyakojima City) education committee. It was a rich spring, and was used only by the government officials at the time. There was a guard posted at the spring. It was the duty of women to go down to the caves and draw water, and since the Meiji era, girls went down to the caves and draw water, as adult women had other duties. Due to the increase in population, it became a business to sell water