Last week's storm brought some of the heaviest rainfall in decades.

By my rough calculations, a little more than 8 trillion gallons fell on the island of Oahu over a 12-hour period.

Although this is a lot of rain, it is only enough to supply the island for about 50 days even if all of it could be captured, which it cannot.

All of that water has to go somewhere, and unfortunately when it falls heavily over a short time like that, most of it runs off the surface. It collects in larger and larger streams, gushes through existing valleys and eventually emerges onto a flat surface.

Only that fraction that seeps into the ground will make it into the water supply. Unlike many continental locations with large reservoirs, almost all of the public water on a small island comes from ground water. This is the technical term for water that percolates through permeable rocks and eventually reaches the water table to recharge the fresh-water system.

Because the flow of ground water is slow (on the order of an inch per day), it does not immediately flow through the rocks beyond the shoreline and into the ocean.

It actually floats on salty ground water like an iceberg of fresh water floating in sea water.

The difference is that the rocks are riddled with cracks and holes and act like a huge sponge. The water flows slowly enough to reach a gravitational equilibrium with the salt water below it.

It forms a lens-shaped mass that, like an iceberg, floats with most of its mass below the saturated saltwater surface that would be at sea level if there were no rainfall.

Whereas ice is 90 percent as dense as water and floats with 9 feet below the water for every foot above, fresh water is 97.5 percent as dense as salt water and floats with only one foot above for every 40 feet below.

The top of the freshwater lens is several hundred feet above sea level and extends deep into the salt water in the spongy rock below the island.

It is from this freshwater lens that Oahu gets most of its drinking water, but only about one-fourth of the rain that falls eventually winds up in the lens.

More than half of it runs off in streams, eventually reaching the ocean but causing flooding when it rains too hard for the streams to contain it or for it to soak in.

A good percentage of it evaporates to return into the atmosphere as water vapor and is carried away by the wind.

A long drizzle allows more of the water to soak in, but those conditions are rare on subtropical islands.

Although enough water soaked in from this past storm to supply Oahu for two weeks, it will take 100 years to infiltrate into the lens and be available.

With this in mind, don't forget to turn off the faucet when you brush your teeth.