Ancient Hawaiians had a saying: “Hahai no ka ua i ka ulu la‘au,” meaning “The rain follows the forest.”

Experts say it’s an acknowledgement that key to maintaining the islands’ supply of fresh water is the forest and its ability to act as a sponge to soak up rain and recharge the aquifers from which we draw our drinking water.

Today, the guardians of these forests are the state’s 10 watershed partnerships, a coalition of 74 public and private landowners and partners on five islands, working to protect more than 2.2 million acres of forested watershed, or about half the state’s lands.

“All the water that comes out of the faucet has to come from somewhere,” said Katie Ersbak, Watershed Partnerships Program planner with the state Department of Land and Natural Resources. “The rain falls to the ground and the native forest is there to capture it and store it in our aquifers and streams and rivers.”

Ersbak spoke at a news conference Wednesday highlighting DLNR’s conservation efforts in conjunction with the World Conservation Congress, the premier conference of the International Union for Conservation of Nature, being held Sept. 1-10 in Honolulu.

Watersheds and their importance in the age of climate change will be the subject of a Sept. 4 workshop at the conference taking place at the Hawai‘i Convention Center.

DLNR’s Division of Forestry and Wildlife manages just over 1 million acres of public land, of which about 900,000 acres fall within watershed partnership boundaries.

Hawaii’s watershed partnerships date back to the early 1990s and are proof that cooperating across landscapes and landowner boundaries is the most cost-effective way to maximize watershed protection, officials said.

Watershed partnerships cover nearly all of Hawaii’s major mountaintops and are usually home to the highest rainfall figures and greatest biodiversity featuring many of the islands’ rarest plants and animals.

The DLNR is currently going through its annual process of awarding $2.5 million in state funding to the watershed partnerships and other groups engaged in watershed protection and management.

Officials say maintaining the health of Hawaii’s forests is more important now than ever in the face of climate change and declining rainfall.

Despite the importance of these wilderness areas, partnership officials say they must hustle to survive.

“One of the challenges is trying to sustain ourselves year in and year out,” said Tracey Gotthardt, operations supervisor of the Koolau Mountains Watershed Partnership. “I spend probably 50 percent of my time chasing funds, either writing grants or talking to prospective granters.”

The Koolau range is the main water source for the Pearl Harbor aquifer, which supplies more than 60 percent of Oahu’s municipal water.

Gotthardt’s crews, like their counterparts in the other partnerships, work in extreme locations under difficult and often dangerous conditions in a battle against alien plants and destructive goats, deer, sheep, pigs and cattle.

Left alone, she said, the Koolau would decline under the onslaught of invasives. Streams and ocean outlets would be muddier as erosion would accelerate. Aquifers would receive less water as a multilayered native canopy would turn into single-story canopy unable to soak up as much rainwater.

The U.S. Geological Survey has estimated that if all the invasive plants were removed from East Hawaii and replaced with native vegetation, the island would gain an additional 85 million gallons a day, said Emma Yuen, acting manager of DLNR’s Division of Forestry and Wildlife Native Ecosystems Program and Management.

Ersbak said that over the last five years the watershed partnerships across the state have planted 120,000 native trees, logged 16,000 hours of volunteer service, provided 80-plus jobs per year and raised $15.4 million of non-state money, matching the funds allocated to them from the state.

“Thank you to watershed partnerships for working with the state to plan and carry out our effort to engage local communities and boost the health of forest watersheds,” Gov. David Ige said in a news release issued Wednesday. “There is renewed hope and measurable success in achieving resilient, healthy forest ecosystems for future generations.”