How do they do this? Tree canopies intercept rain, snow and other forms of precipitation. In so doing, they both decrease the impact velocity of a rain drop hitting the ground, and reduce the overall amount of precipitation that eventually reaches the ground. The faster the moving water, the larger the particles are that can be carried away and the more severe the erosion.
Water that is stored in the tree canopy is returned to the air by evaporation or transmitted to the ground for root absorption. The tree uses some of the absorbed rainfall and eventually releases the unused portion back into the atmosphere through a process called transpiration. In addition, leaf litter under the tree serves as a sponge for the water. The result of this moderating effect on precipitation is a reduction in runoff and soil erosion.
Here are some examples of how trees pay us back:
• Urban forest can reduce annual stormwater runoff by 2–7 %.
• Green streets, rain barrels, and tree planting are estimated to be 3-6 times more effective in managing stormwater per $1,000 invested than conventional methods.
• Implementing green infrastructure practices in Detroit’s sewage and water department will reduce combined sewer overflow volumes by 10-20% and reduce annual costs by $159 million a year.
• Portland, OR, is saving 43% ($64 million) by integrating green infrastructure - including planting 4,000 trees - into a combined gray-green stormwater management solution rather than the standard man-made approach.
• Street trees in Minneapolis saves the city $9.1 million in stormwater treatments annually.
• Philadelphia’s $1.5 billion stormwater management plan focuses almost exclusively on eco-friendly solutions - bioswales, permeable pavement, street trees - as a way of reducing the city’s 15 billion gallons of annual water overflow.
• The stormwater management value of Philadelphia’s parkland and trees is $5.9 million annually
• Trees on UC San Diego's 1,200-acre campus trap and filter nearly 140 million gallons of storm water runoff each year at a value of $250,000.
• Urban greening in Washington, DC, prevents over 1.2 billion gallons of stormwater from entering the sewer system, 10% of the total volume. This represents a savings of $4.74 billion in gray infrastructure costs per 30-year construction cycle.
• Trees in Houston, TX, provide $1.3 billion in stormwater benefits (based on $0.66 /cubic foot of storage).
• Each urban tree in Modesto, CA, reduces stormwater runoff by 845 gallons annually, with a benefit valued at $7 per tree.
• Street trees in New York City intercept 890 million gallons of stormwater annually: 1,525 gallons per tree on average, with a total value of over $35 million each year.
What can you do?
- Maximize the amount of growing space and understory vegetation around a tree.
- Preserve established trees and minimize soil compaction, displacement, and erosion around a tree.
- Minimize clearing of trees and vegetation to preserve their benefits and minimize soil compaction.
- Do not over fertilize or over irrigate your trees or lawns.
- Route excess stormwater to bioretention areas made of a vegetated buffer and a soil bed to filter pollutants, store water and prevent erosion.
- Include tree and vegetative strips in parking lots to collect, store and treat the runoff.
- Maintain and increase the amount and width of urban forest buffers around urban streams, lakes and wetlands