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water table
Figure I : High island (half width) showing emergent volcano with feeder
flanking coral reef. Rain water is shed by direct runoff, or
compartments in the volcanics or to the fresh-water lens.
Hydrology
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ter tabte
dikes and sills, and
percolates to water
Oceanic islands' water resources are derived entirely from rainfall. The volcanics of
the high islands are relatively impermeable and much of the precipitation is shed as surface
runoff via streams. That part which penetrates the rock to the water table can be perched on
confined to ground-water compartments formed by dikes, sills, or flows. Leakage from these
confining beds allows ground waten to percolate vertically or laterally Lo lower level, and
ultimetely it replenishes a fresh-water lens (descnibed below), the surface of which might be
a few centimetres asl.
The surrounding reef, parl of which may be up to 70 m asl, has a karst topography and
water falling on its surface, and that shed from the interior volcanics moves rapidly through
fissures, caves, solution channels and cracks to the fresh-water lens.
(1)
Water Resources
Fresh water is obtained from a number of sources on high islands by:
Roof Catchment. Individual household tanks, or community tanks filled by
specially-designed, roof -catchment structures.