The Hawaiian Rainforest

The Ultimate Watershed

"Water collection is an essential function of all forests, but the Hawaiian forest seems perfectly designed by nature for the task."


When you live on islands in the middle of a vast ocean, it’s hard to put a price on the value of fresh water. Without it there is no life. The Hawaiian Islands lie in a relatively arid part of the Pacific, 2,500 miles from the nearest continental landmass, and rainfall in the surrounding ocean is only 25 to 30 inches a year. Yet freshwater is abundant here. Where does it come from?

The answer lies in the convergence of winds upon the Islands’ richly forested mountains. As warm ocean air moves inland, it is forced upwards by the mountainous terrain, cooling and condensing on its forested slopes. The upland forest captures water in the form of mist, fog and rain, absorbing and releasing it into streams and underground aquifers.

Water collection is an essential function of all forests, but the Hawaiian forest seems perfectly designed by nature for the task. Its dense canopy provides an umbrella that intercepts rain. Its thick layered understory acts as a giant sponge, soaking up water. Its roots grip the mountain and anchor the soil, reducing erosion and enhancing surface water quality.

Millions of years of evolution have made the Hawaiian forest highly efficient at capturing and retaining water. Generally speaking, the more complex the structure of a forest, the more enhanced its watershed functions.

With its marvelous multi-layered structure—tall canopy, secondary trees, shrubs and fern layers, ground-hugging mosses and leaf litter—the Hawaiian rainforest acts like a giant sponge, absorbing water and allowing it to drip slowly underground and into streams.

Even without rain, the forest can pull moisture from passing clouds. In Hawaiʻi, this interception can push water capture above and beyond total annual rainfall by as much as 30 percent.

A Conserver of Water

But the Hawaiian forest doesn't just capture water. It also conserves it. The tall, closed canopy shades out the sun, resulting in less water lost to evaporation and transpiration. The dense vegetation blocks wind, which would otherwise pull moisture from the land.

The many layers of vegetation blunt the erosive effects of rain, and, once saturated, buffer the release of stored water, reducing immediate flow in wetter times, maintaining it in dry. Long after the rain subsides, the native forest delivers fresh water for human use.

Finally, there is a direct correlation in Hawaiʻi between the health of our forested watersheds and the health of our reefs and beaches. Without a healthy forest to anchor the soil and temper the erosive effects of heavy rain, large amounts of sediment wash off our steep mountains and into the ocean, polluting streams, destroying coral reefs and degrading coastal fishing resources.

Effective protection of Hawaii's watersheds is of enormous importance. Fresh water is not an unlimited resource, and its ready availability, quality and sustainability are linked to the health of our forested watersheds. When we destroy our forests or allow our watersheds to degrade, we put our future prosperity and quality of life at risk.



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