After rainwater flows over roads, parking lots, and lawns, it picks up fertilizers, oil, chemicals, grass clippings, litter, pet waste, sediment, and other contaminants. This contaminated stormwater flows into underground pipes and local streams and then into local lakes (Mountain Island, Wylie).
The amount of hardscape (roads, buildings, parking lots) in Charlotte has been increasing as the city develops, causing even more runoff that previously would have been naturally absorbed into the ground (i.e. via lawns, parks, fields, woods).
Trees and Forests as a Benefit
Where rain falls on paved surfaces, a greater amount of runoff is generated compared to runoff from the same storm falling over a forested area.
Trees contribute to stream quality and watershed health primarily by
One mature deciduous tree can intercept over 500 gallons of rainwater a year, while a tree that holds leaves all year round (i.e., pines, magnolias) can intercept up to 4,000 gallons per year (Seitz 2008).
Trees are most effective at intercepting water from very brief periods of rainfall. Leaves can hold water until it evaporates. The contribution made by trees is greatest in forested areas, where the trees and underlying earth can work together to absorb the rain.
The presence of trees does not change the ongoing need for municipal and private investment in adequate storm water infrastructure to manage larger storms and enhance water quality.
What Trees are Already Doing
At its current coverage of 47%, Charlotte's trees currently intercept 1.2 BILLION gallons of rainfall each year.
It is difficult to model the economic impact of this benefit because so many factors related to tree type, seasonal variations, soil absorption capacity, and storm size and intensity, are involved. An estimated value of $10 million per year has been selected based generally on Charlotte’s experience with engineered rainfall detention strategies.
Charlotte's trees currently intercept 1.2 BILLION gallons of stormwater each year. This service is valued at $10 million annually to the city.