Geology and the hydrological cycle

The Shimajiri mudstone layer of bedrock is an impermeable rock with an average hydraulic conductivity of 2 x 10-6 cm/sec. Over this, the highly permeable Lyukyu limestone layer with a hydraulic conductivity of 3.5 x 10-1 cm/ sec forms the aquifer with a thickness of between 10 and 70 m. The effective porosity of the Lyukyu limestone is estimated at between 10 and 15 per cent. Tectonic movements accompanied by several faults have formed underground valleys, and groundwater flows along these valleys.

Thus, 40 per cent of the abundant rainfall on Miyakojima penetrates underground and quickly flows 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 can flow along the surface (fig. 2). Using the water resources on Miyakojima has been quite difficult, owing to these natural conditions.

Figure 2 The Hydrological Cycle on Miyakojma and the Overall Average in Japan

Before construction of the drinking-water supply system in the 1960s, obtaining drinking water was hard work for the women and children, owing to the above natural conditions. Agriculture, the main industry on the islands, still experiences frequent drought.