Anywhere from 20 to 90% of the precipitation that falls on forests returns to the atmosphere through “evapotranspiration.”

Evapotranspiration involves two processes: evaporation of water into the air from the forest canopy and soil; and transpiration of water into the air from trees through the stomata in their foliage.

The water that doesn’t return to the air takes one of two paths: it is stored temporarily in the forest soil before becoming runoff that feeds streams and lakes; or it percolates deeper down through the soil to become groundwater.

The drinking water for two-thirds of Canadians comes from surface waters, including lakes and reservoirs. A large proportion of the water in those sources originates in forested areas.

Role of forests in water health

Forests are an integral part of the water cycle.

  • The chemical makeup of water, as it passes through a forest ecosystem, changes. It gets cleaner because the soil filters out substances such as mercury, pesticides and other pollutants.
  • The forest cover slows down erosion and delays the release of water into streams, helping stabilize the quality and quantity of water in the area.
  • Forests recharge and maintain the quality of groundwater.

Therefore, one of the most important benefits that forests deliver is a reliable supply of clean water. For this reason, researchers monitor threats that could upset the delicate forest-water balance and seek ways to minimize those threats.

A large-scale environmental laboratory

Since 1980, the Turkey Lakes Watershed north of Sault Ste. Marie, Ontario, has been the subject of a study about acid precipitation effects on forest ecosystems. Researchers from Natural Resources Canada, Environment Canada, and Fisheries and Oceans Canada, along with many other partners, have gathered 30 years of data on stream and lake chemistry, hydrology, meteorology, soil chemistry, tree growth, fish, and benthic invertebrate populations.

The result is one of the longest-running, most comprehensive data sets in North America.

The Turkey Lakes Watershed study has shown the effectiveness of air pollution regulations, including the Canada-U.S. Air Quality Agreement, in preventing acid precipitation. The study has also probed the relationships among climate variability, bio-geochemical cycling and biological response. In 1997 a forest harvesting experiment began in the area to investigate the impacts of variable harvesting intensity on forest productivity, soil processes, and water quality and quantity.