Puget Sound Urban Tree Canopy and Stormwater Management - iTree-Hydro-Technical-Report_Revised-March-2019.pdf

and development in
the region’s most populous counties (King, Pierce, and Snohomish)
has led to an expansion of impervious surfaces, which in turn
increases stormwater runoff and degrades water quality from
pollutant loading. Comprehensive restoration plans and policies
have been developed to address water quality degradation
associated with urban development and increased stormwater.
The Puget Sound Urban Tree Canopy and Stormwater Analysis
Project is designed to enhance the discussion around those plans
and policies, with information about the role tree canopy plays in
managing water quality and quantity in the Puget Sound region.

Key findings from the comparison, presented in further detail in this report include:
1. In nearly all modeled scenarios, i-Tree Hydro yielded lower runoff volumes, and therefore a lower benefit of tree canopy than WWHM.
2. Increase in tree cover over impervious surface results in decreased runoff volumes.
3. Development that includes tree retention results in reduced runoff volume compared with development without tree retention.
4. Scenarios where tree canopy is replaced with any other land cover type, including herbaceous layers or impervious area, result in
increased runoff volume.
5. Areas with higher existing tree canopy coverage experience a lower magnitude of runoff volume when tree canopy is reduced.

With expertise from urban forestry program and policy managers, supplemented by analysis from i-Tree Hydro, decision
makers can more effectively factor in the impact of a decrease in tree canopy and an increase in impervious surfaces in the planning stages
of development projects involving changes to land cover. Including tree canopy in the discussion opens the door for forward-looking
strategies, including forest retention and canopy enhancement as a complement to mitigation strategies in a particular jurisdiction