Forests Supply Water to Puerto Rico

Report quantifies water yield from island's tropical rainforests

Puerto Rico’s forests are still recovering from Hurricane Maria. Local scientists estimate that one out of five trees in the El Yunque National Forest were lost during or after the storm.

El Yunque, once known as the Caribbean National Forest, is home to 23 plant and tree species that are found nowhere else. USFS photo.

Widespread forest restoration and monitoring efforts are underway. But that work isn’t focused on just the trees in those forests — water supply is an important ecosystem service.

“With the Forests to Faucets project in 2014, we learned that national forests supply drinking water to 19 million people in the Southeast,” recalls USDA Forest Service scientist Erika Cohen Mack. Mack led an effort to update the Forests to Faucets analysis with estimates for the U.S. commonwealth of Puerto Rico.

The Caribbean island is home to more than 3.7 million people and about 835,000 acres of forest land. At 28,000 acres, El Yunque is the smallest national forest and only tropical rainforest in the National Forest System.

The research team wanted to know how the El Yunque, along with other forest lands – either commonwealth or privately owned – contributed to the island’s drinking water supply.

They replicated the methods that they used for quantifying water use from forests across the South, using the improved WaSSI ecosystem services model, this time updating its inputs and equations to accurately capture Puerto Rico’s unique environment.

By spring, palm trees had begun to sprout new leaflets and the island landscape shows signs of greening. Photo by Steve McNulty, USFS.

“WaSSI is a monthly water balance and flow routing model developed by SRS scientists,” says Mack. “The model simulates hydrology components at a small basin scale, including surface runoff, baseflow, infiltration, and soil storage. Then it routes the water to estimate total flow from a headwaters area, through a network of downstream drainage basins, to the mouth of a river.”

Mack, along with SRS research hydrologists Ge Sun and Peter Caldwell, visiting scholar Liangxia Zhang, and Southern Region hydrologist Suzanne Kreiger, published their results as an SRS General Technical Report Addendum.

They learned that a drainage basin downstream from the El Yunque provides about 50 percent of the water supply for the San Juan metro area. The basin also provides about as much for 12 other municipalities downstream from the national forest.

The total water supply estimated to come from national forest land was about 219 million cubic meters per year or just over three percent of the island’s total water supply.

Commonwealth, or public, forests and private forest lands provided water to 78 municipalities in Puerto Rico, serving a total population close to 3.6 million with 2.6 billion cubic meters per year, about 40 percent of Puerto Rico’s total water supply.

Rainfall varies across the island, from 30 inches per year in southwestern areas, where the Maricao State Forest is located, to 170 inches per year in windward, eastern areas. Photo courtesy of Wikimedia Commons.

“Forests make up about 38 percent of Puerto Rico’s total land area and contribute approximately 43 percent of its surface water supply,” adds Mack. “Forests are critical to the island’s water supply.”

Add that to the other benefits that they provide — habitat for endangered species like the Puerto Rico parrot, idyllic settings for recreation and tourism, and a host of other natural and cultural values shared by residents and visitors alike.

Zhang led another study that quantified the size and timing of hydrologic fluxes. This complementary work updated evapotranspiration estimates with eddy covariance data from rainforest ecosystems. The results were published in Forest Science.

At least 60 percent of Puerto Rico’s public and private forest lands are protected from conversion to other land uses. This protection — along with ongoing forest restoration efforts — will help ensure that the “island of enchantment” can provide clean drinking water for generations to come.

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For more information, email Erika Cohen Mack at

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