The water cycle on an island follows the same principles and properties of water anywhere, but ocean island geology and geography create unique details in how the cycle plays out.
The island water cycle begins with ocean evaporation. Moist air cools as it rises and as the humidity level of this air increases to 100%, water vapor condenses to form clouds. Most rainfall in Hawai`i results from orographic lifting, the lifting of clouds as they're pushed up against the islands’ central mountains by northeast tradewinds. As prevailing winds push the moisture-laden air to approximately 2,000 meters, the air reaches its saturation point where cloud vapor condenses to water and rain results.
The areas with the greatest rainfall are also those areas with the most persistent uplifting, that is, areas on the windward sides of the islands. Large valleys form under the most intense rainfall. Rain shapes small gullies in areas of less intense rain. Leeward plains can remain essentially uneroded; rainfall comes there only when the heaviest storms push clouds over the mountains.
Erosion from surface streams shapes valleys. Because of each island's conical shape, streams are steep and closely spaced in the interior, and more widely spaced at the shoreline where streambed slope is also more gentle. Over time, each island has been cut into triangular pieces, with wide-mouthed valleys converging in the island center, the valleys separated by narrow, steep ridges.
Rainwater also seeps through soil and rock and percolates down to each island's underground aquifer. The structure of Hawaii's lava flows results in islands made of porous rock that is highly water-permeable. Island rock below sea level is permeable to sea water in the same way upper rock levels are permeable to fresh water. Where they meet, the less dense fresh water sits atop the sea water in a distinct layer. An intermediate zone is mixed, or brackish, water.
Fresh water discharged from the aquifer through springs is constantly replenished by percolating rainwater. Fresh water entering the ocean through springs and streams is balanced by water evaporating from the ocean's surface. The entire cycle is in constant flux, maintaining an equilibrium through motion, transformation and replenishment.
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