How Do Trees Clean Water?

Key takeaways

• Forests are a source of clean drinking water for more than 150 million people in the United States, according to the USDA Forest Service.
• Forested stream buffers have cut nitrate by up to 88 percent and phosphorus by up to 76 percent in farm runoff.
• Protecting the forested Catskill and Delaware watersheds has spared New York City a filtration plant estimated to cost up to $10 billion.
• Plantations on land that was not previously forested decreased stream flow by 52 percent in a global analysis, so where trees go matters as much as how many.

Roughly 1 billion gallons of water a day reach the 9.5 million people who drink New York City's tap water, and about 90 percent of it begins upstream in the Catskill and Delaware watersheds, traveling more than 175 miles of aqueducts and tunnels from these upstate forests to the city's edge. By protecting the land around its reservoirs, the city has put off building a filtration plant estimated at up to $10 billion. "Preserving forested lands that surround unfiltered reservoirs is the primary line of defense for drinking water protection," Eric Goldstein of the Natural Resources Defense Council wrote. "It's a remarkable success," Jay Lund, a University of California, Davis, engineering professor who served on a 2020 National Academies review of the program, told Circle of Blue.

New York's arrangement is one of the best-documented examples of a relationship that plays out in watersheds everywhere, including the hill country of northern Thailand, where we at One Tree Planted support restoration work.

How Do Trees Clean Water?

Trees clean water by intercepting rainfall in their canopies, slowing its path into the soil, and filtering it through their roots, which hold back sediment and absorb pollutants before runoff reaches a stream. Forest canopies alone intercept 10 to 40 percent of rainfall, depending on the species and the storm, and a mature evergreen can catch more than 4,000 gallons in a year.

How trees absorb and move water

Water enters a tree through its fine roots, where root hairs and partnerships with mycorrhizal fungi widen the absorbing surface, then passes through several layers of root cells before reaching the xylem, the open tubes that carry it upward, plant physiologists explain in Nature Education. "These cell layers act as a filtration system in the root and have a much greater resistance to water flow than the xylem, where transport occurs in open tubes," the authors wrote. "Imagine the difference between pushing water through numerous coffee filters versus a garden hose."

Trees have no heart to pump that water skyward. Evaporation through tiny pores on the leaves, called stomata, creates negative pressure that pulls a continuous column of water up from the roots, a process scientists call the cohesion-tension mechanism. Water moves 92 feet per hour through a red oak, 11 feet per hour through a basswood, and 6 feet per hour through a pine, Lindsey Purcell writes in Purdue University's Landscape Report. A tree can absorb between 10 and 150 gallons a day and releases nearly 95 percent of it back into the air as vapor, rehydrating from the soil overnight.

How tree roots filter stormwater

Rain falling on pavement collects everything on the surface, such as gas and oil, heavy metals, sediment, pesticides, and fertilizers on its way to the nearest storm drain, Vincent Cotrone, an urban forestry extension educator at Penn State, explains in a Penn State Extension video. Tree roots open channels that let water sink into the ground instead. In one study he cites, soil with trees absorbed 10 inches of rain per hour, while nearby soil converted to lawn absorbed four. Roots also take up heavy metals and other chemicals from contaminated runoff, a process known as phytoremediation.

Why Trees Matter for Watershed Health

A watershed is the area of land that drains rain and snowmelt toward a single stream, river, lake, or wetland, and forests remain the most reliable cover for keeping that water clean. "More than a century of research has demonstrated that forested lands provide the cleanest and most stable water supply compared to other lands," Cynthia West, then acting deputy chief of Forest Service Research and Development, wrote for the USDA.

Streamside forests, known as riparian buffers, do much of that work. Along with trapping nitrogen and phosphorus, they stabilize banks and shade streams, which moderates water temperature for fish and other aquatic life. Paved land behaves very differently, since one acre of parking lot generates as much annual runoff as 36 acres of forest, per Penn State Extension.

Forest cover in the world's major watersheds is shrinking. Across 230 watersheds, tree cover fell 6 percent on average between 2000 and 2014, according to the World Resources Institute. Researchers surveyed more than 75,000 bodies of water in 89 countries and found more than 40 percent severely polluted. The number of people at risk from floods is projected to reach around 1.6 billion by 2050, nearly 20 percent of the world's population, per the UN's 2018 World Water Development Report.

Where We're Restoring Watersheds

In the hill country of northern Thailand, near the border with Myanmar, we have supported a partner working with hill tribe communities to plant 375,000 more trees within the Nam Kohk, Hoi Lui, and Mae Na Wong watersheds. The planting is designed to rebuild soil, restore degraded habitat, and improve local water sources, and it supports village and family cisterns of reinforced concrete that collect stream water in the wet season for use through the dry months.

Tree Planting Risks and Challenges for Water

Trees drink, and planting them where forests did not grow naturally can leave less water for everyone downstream. A 2005 analysis of more than 600 observations found that plantations decreased stream flow by 227 millimeters per year (52 percent), with 13 percent of streams drying completely for at least one year, alongside increased soil salinization and acidification. "It doesn't matter where you are in the world, when you grow trees on croplands, you use more water," Robert Jackson, the study's lead author, then of Duke University, told Nature. "Policy-makers often have a set of 'carbon blinders' on - they're thinking and talking only about carbon."

The study looked at afforestation, or planting trees on land that was not previously forested, which is why species and site selection carry so much weight in watershed restoration work that aims to bring back forest where it once grew. "For too long, the world has turned first to human-built, or 'grey,' infrastructure to improve water management,” Gilbert Houngbo, then chair of UN-Water and president of the International Fund for Agricultural Development, wrote in the foreword to the UN's 2018 World Water Development Report. “In so doing, it has often brushed aside traditional and indigenous knowledge that embraces greener approaches." 

"If we do nothing, some five billion people will be living in areas with poor access to water by 2050," Audrey Azoulay, director-general of UNESCO, added in the same report.