By Jennifer Teegarden, MDNR Cooperative Forest Management Outreach Specialist and MNWWN Board Member
Keeping trees and forests on the landscape is the easiest way to produce clean water—for drinking, recreation, and wildlife habitat. Studies show protecting watersheds upstream from a city is cheaper than building treatment plants to produce clean drinking water (1). For example, a survey of 27 water suppliers in 2002 by the Trust for Public Land and the American Water Works Association found that for every 10 percent increase in forest cover in a source watershed, treatment and chemical costs decreased by about 20 percent.
The soils of a forested watershed is a reason why they produce clean water. Forest soils have numerous and large pore spaces like a sponge, which allows water to move easily into the soil. As a result, more water will recharge aquifers and less will run off the forest floor, which reduces flooding of rivers and erosion of riverbanks. The large pore spaces in the soil also trap sediment, nutrients, and pollutants, like a filter. Thus, water leaving the forest and entering water bodies is cleaner. This means cleaner water for aquatic wildlife and less development of toxic algal blooms that are harmful to our pets.
The canopy of trees, and the vegetation growing underneath, intercepts rain as it falls through a forest, like an umbrella. This slows down the time and the speed in which a raindrop hits the forest floor. Consequently, the potential for flooding is reduced because the amount of time it takes runoff to leave a forest and enter a waterbody is lengthened. Additionally, water leaving the forest is cleaner because slower moving raindrops dislodge less sediment when they hit bare soil.
Trees in the urban landscape also keep our water clean, along with managing storm water, which is a cost benefit to us. For example, according to the National Tree Benefits Calculator, a 15-inch diameter hackberry growing in a single-family residence in Minneapolis will intercept 1,427 gallons of storm water this year, a cost savings of $38.66. Increasing the amount of canopy in an urban environment will reduce the amount of storm water entering and overwhelming a city’s sewer system. This is especially important in areas receiving more rainfall amounts during storm events due to climate change.
Not all trees are alike when intercepting rain. Trees with more leaf area and year-round foliage, such as conifers, intercept more rain. Larger trees with broader canopies with branches parallel to the ground will intercept more rainfall. If you have the space, create a “mini” forest by planting shrubs and groundcover under a large tree. Be sure to plant a variety of trees so your urban forest can adapt to a changing climate and be resilient to future invasive tree pests.
1. Center for Watershed Protection: Forests and Drinking water