What is a Watershed?

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.

The Hawaiian equivalent of a watershed is the ahupua‘a. In Hawaiian cultural tradition, an ahupua‘a is a land division with the streams and valleys serving as boundaries, varying on different islands from as little as 100 acres to more than 100,000 acres. Ahupua‘a included the land from the mountains to the coast, and the coastal ocean extending out to and including the coral reef. Native Hawaiians also recognized the importance of forests in water production, described in this proverb: “Hahai nō ka ua i ka ulu lā’au” – Rain always follows the forest (click here to read more about linkages between forested watersheds and Native Hawaiian cultural resources).

“In Hawai‘i, the most valuable product of the forest is water, rather than wood.”
— Ralph S. Hosmer, First Territorial Forester