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How does it work?Rainfall is only a part of the water cycle equation. Our rainforests and watersheds are the island's major water producers. Trees, shrubs, and mosses capture rainwater and can actually increase rainfall and condensation. Forested watersheds perform four basic and crucial functions. We call these functions the Umbrella, Anchor, Sponge and Pump. UMBRELLATree leaves, branches and understory plants intercept rain before it reaches the ground. This slows down the velocity and force with which the water hits the soil. The "umbrella" reduces the rain’s ability to erode soils and increases the infiltration of rain water into the ground. Fog condensing on trees and other vegetation is an important part of water resources. In Hawai’i, fog interception can increase total annual rainfall by as much as 30%! SOIL ANCHOR
For example, the picture above is a stand of Miconia, a fast growing tree from South America that is now invading Hawai’i. It shades out other plants, meaning native plants and seedlings do not get enough sunlight to grow. A forest of miconia does not anchor soil as well as a native forest, and soil is easily eroded. Compare this picture with the one below of native forest. SPONGE
Can an urban watershed act as a sponge? No, not very well. Most cities are paved with concrete and cement, which are not very porous. Little or no water is absorbed into the soil beneath; most of the water runs down streets and into gutters. The people of Maui must depend on the water saved from forested watersheds to supply fresh water in dry times. PUMPPlants use water, which is released back into the atmosphere through evaporation and transpiration (evapotranspiration). Both of these processes are increased by warm and sunny conditions. The cooler temperatures and cloudy conditions found in the East Maui Watershed greatly suppress evapotranspiration. This allows much of the rainfall and condensed fog to soak into the ground and move through the soil. Less water is “lost” into the atmosphere and more water is retained. Thus, forested watersheds “pump” water back into the soil, which appears later as clean water in underground and surface streams.
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How well do you know the water cycle?
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The
roots of trees and plants grip the steep mountain soils of the East Maui
Watershed, preventing it from washing into the ocean. This protects
our ocean reefs and marine life. Invasive plants can threaten the anchoring
ability of a native forest. 
A
forested watershed acts like a sponge, soaking up rainfall into its soil,
roots, mosses, ferns and leaves. When they are all fully saturated, they
slowly release water, thus delivering a consistent and dependable source
of water for eventual use by the forest, wildlife, and humans, long after
the rain has fallen. 
