How much of a watershed must remain forested to produce clean water? And what kind of information do water utilities, government agencies, and other funders need to support further investment in land protection as a strategy to maintain water quality? Answering these questions is essential to keeping clean water clean.
Despite broad scientific consensus on the importance of forest cover to maintain water quality, there is a tendency among public agencies and funders to privilege “fixing things” over efforts to avoid impacts in the first place. This emphasis leads to high spending to restore degraded farmland and misses an opportunity to protect intact forests now so that they don’t become degraded and cost a great deal more to restore later. For example, in 2018 the federal government allocated $73 million for restoration in the Chesapeake Bay and only $6 million for land conservation, a 12 to one ratio. Nationwide, the amount that water utilities in the United States spend annually to chemically treat drinking water is 19 times what the federal government invests in land protection that keeps pollutants out of lakes and rivers.[1]
A new brief summarizes key findings from two reports conducted by Open Space Institute (OSI): Literature Review: Forest Cover and Water-Quality Implications for Land Conservation (Morse et al. 2018) and Water Quality Protection Programs: Insights from Six Eastern United States Cases (Morse and Weinberg, 2019) and offers recommendations for addressing gaps in knowledge that clarify the case for land protection as a high impact investment to safeguard water quality.
For Clean Water, How Much Forest Cover?
Watersheds with 60-90 percent forest cover reliably yield high water quality, but forest location and the type and intensity of other land use also matter.
Land use studies from around the globe affirm that abundant forest cover is crucial to water quality. OSI’s research confirmed through review of the scientific literature that forest cover between 60 and 90 percent has been proven to meet a variety of ecological and chemical definitions of water quality.
Notably, major metropolitan centers that meet the Environmental Protection Agency’s filtration avoidance criteria, which allow cities to use the filtration provided by forests in lieu of building multibillion-dollar water treatment plants for their public water supply, maintain forest cover within this range (see Table 1).
Table 1. Forest Cover in Select Watersheds Serving Metropolitan Centers
Watershed |
Water Supply |
Percentage Forest Cover |
Catskill/Delaware |
New York, NY |
75% |
Quabbin |
Boston, MA |
88% |
Sebago Lake |
Portland, ME |
84% |
The exact amount of forest cover needed depends on a range of factors that are broad but knowable. The non-forest land uses within a watershed are a significant factor – for example, is the remainder of the land cover in low-intensity agriculture or high-density development? If the latter, more forest cover will be needed to slow down fast-moving water running off impervious surfaces. Our review confirmed that water quality can begin to degrade when a forested watershed exceeds 18-50 percent agricultural area or as little as 3 percent impervious area.
Other variables, like the location of forest cover relative to water bodies or highly erodible soils, can also be influential. Literature confirmed the importance of protecting forests buffering streams in the headwaters of watershed. In sum, the scientific literature suggests that the more forest cover in a watershed—especially in sensitive areas like steep slopes—the better for water quality.
Non-Forest Land Uses: The type and intensity of non-forest land use in a watershed have different impacts on water quality. Water quality can begin to degrade when a forested watershed exceeds 18-50 percent agricultural area (left), or as little as 3 percent impervious area (right).