A watershed is an area of land, such as a mountain or valley, which collects rainwater into a common outlet. In Hawai‘i, the common outlet is ultimately the ocean. Some of the rain is absorbed by plants, some of it is absorbed underground, and the rest flows into surface rivers and streams. A critical component of a watershed’s ability to collect rainwater is the existence of forests. Fog condensing on trees high up in watershed areas can increase
rainfall collection and absorption by as much as 30% annually.
“In Hawai‘i, the most valuable product of the forest is water, rather than wood.”
— Ralph S. Hosmer, First Territorial Forester