Trees and Stormwater Management

1. Trees and Stormwater Management Promoting forest preservation and restoration at development sites through stormwater credits

2. Goals 1) Preserve forests and natural vegetation at development sites 2) Protect and enhance urban and suburban tree canopy 3) Increase the use of trees in stormwater practices Photo by Jay Wilson

3. The Chesapeake Bay Program has formally recognized the benefits of trees for stormwater management • WE FURTHER RECOGNIZE THAT URBAN TREE CANOPY COVER offers stormwater control and water quality benefits for municipalities in the Chesapeake Bay watershed and can extend many riparian forest buffer functions to urban settings. (Chesapeake Executive Council, Directive No. 94-1, Riparian Forest Buffers)

4. Old News • The first models for predicting runoff appeared in the 1800s and used runoff coefficients to account for different land use and land covers. • The Rational Method (Mulvaney 1851) • Peak Discharge = Runoff Coefficient x Rainfall x Basin Area • Runoff Coefficients: • Forested ground is typically assigned a value of near zero. • Pavement is given values approaching 100 percent. • But why emphasize trees in particular versus other stormwater practices?

5. Meets Multiple CBP Directives • Promote expansion and connection of forest habitat • Chesapeake Bay 2000 • Minimize forest loss and fragmentation. • Directive 94-3: Framework for Habitat Restoration • Increase urban and suburban tree canopy cover • Directive 94-1: Riparian Forest Buffers • Promote innovative stormwater BMPs • Directive No. 98-3: Accelerating Bay Restoration Through implementation of Innovative Technologies

6. Premise: Trees provide so many benefits beyond stormwater that they should be a priority consideration in site design. • Unlikely to happen unless trees are given credit for their work. • Stormwater is one place where we can give them credit. USDA Forest Service Urban Watershed Forestry Manual, Part 1

7. Don’t trees get some credit already? • Not in most municipalities • Not enough to matter to most developers. • Not enough to equal their true value Source: CWP (borrowed from one of Tom Schuler’s presentations)

8. Figure 1. Schematic of a Tree’s Hydrologic Cycle (Source: FISRWG, 1998) Ways that trees improve water quality • Three main ways: • Rainfall interception • Evapotranspiration • Infiltration • The overall effect: • Delay peak runoff during storms • Increase soil infiltration and groundwater recharge • Remove nutrients and other pollutants • Decrease volume of stormwater that needs to be treated, especially from small, frequent storm events. • “That’s great, but can you give me some numbers?” • How else can our engineers calculate credits?

9. A Few Examples from the Scientific Literature • Rainfall Interception • 10-40% of annual rainfall depending on tree species and climate (Zinke 1967) • Evapotranspiration (ET) • Eastern Forests: 300 – 900 mm (12-36”) (Kittredge, 1948). • Infiltration • Kays, 1982 • 12.4 in/hr for forest, • 4.4 in/hr for lawn • 1.9 in/hr for suburban development (Kays, 1982)

10. That’s fine, but what about MY trees? • New models have been developed to estimate the water quality and quantity function of trees: • STRATUM – Quantifies stormwater volume benefit of street trees (i.e how much runoff volume was avoided via tree canopy interception). • UFORE Hydro – A more advanced model – calculates both stormwater volume and water quality benefits for individual trees and forest patches using interception, infiltration, and evapotranspiration. • Designed for ease-of-use • Simple Windows interfaces (no technical knowledge required) • For municipal foresters, commercial arborists, environmental consultants, planners, etc. • Do we have to run the model for every site? UFORE: Urban Forestry Effects Model STRATUM: Street Tree Resource Analysis Tool for Urban Forest Managers

11. Use models to generate regional Tree Credit guidelines • UFORE Hydro is almost ready. • STRATUM software is written and data for our climate region is coming soon. • We’ll be able to fill out charts like these (below) to create tree credit guidelines.

12. Example Output from STRATUM STRATUM calculates both economic and environmental benefits of trees. (Example above is from the street tree population of North Vancouver)

13. Other approaches being explored • Premise: We have many manuals and publications about LID, but we have seen minimal implementation by developers and in local ordinances. • Issue paper: Obstacles to LID implementation • Based on interviews with developers, local officials, and public works people. • Find out the challenges and misconceptions. • Identify potential solutions. • Compile some successful case studies.

14. Summary of Efforts • Use scientific literature and models to provide “real numbers” for the stormwater quality and quantity impact of trees. • Demonstrate how forests and tree canopy help meet important goals and directives • Cleaner air, lower energy use, habitat enhancement and protection, forest connectivity, carbon sequestration, etc. • Speak with developers and local government officials to find out: • Why more LID practices aren’t used • How to lower the barriers to using these techniques. • Use case studies to highlight successful examples.

15. Seeking Workgroup’s Advice • How can we use this information to affect change (i.e. preserve and enhance tree canopy cover in urban areas and at new development sites)? • Who are the target audiences? • How do we package this information to be most effective? • Final Products? • Tree Credit Guidelines: Use UFORE Hydro and STRATUM models to create regional tree credit charts that state/local governments could use. • Issue Paper: Obstacles to Incorporating Trees/Natural Areas into SW • GOALS REVISITED • 1) Preserve forests and natural vegetation at new development sites • 2) Protect and enhance urban and suburban tree canopy • 3) Increase the use of trees in stormwater practices