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The Peculiarities of Pervious Cover: A research synthesis on allocating pollutant loads to urban land uses in the Chesapeake Bay

Reference Type
Report (Research or Project)

Section 7 (pp 40-44) of this report discusses how tree canopy and forest fragments should be handled/credited on pervious land in the Chesapeake Bay Watershed Model.

Key findings include:

• Trees are known to reduce stormwater runoff through interception of rainfall and increased evapotranspiration (ET), infiltration, percolation, and soil storage

• Modeling tools (i-Tree Hydro) can quantify modest runoff volume and pollutant reductions

o Few studies are available that directly address nutrient and sediment fluxes due to tree canopy

o Hydro uses national EMC averages

• In general, ET of forested areas is greater than that of grass due to greater leaf area/biomass and deeper roots o Minnesota developed a stormwater credit for ET that is tied to canopy size

o Credit is based on anticipated mature size with sufficient soil volume (1ft2 canopy per 2 ft3 soil)

• Organic matter from tree leaves can increase nutrient loading in receiving waters, but this material is beneficial to stream biota

o It is recognized that there is an absence of published data on this topic

• Soil volume and quality affect tree size, growth rate, and overall effectiveness of tree canopy

• Much of a city’s tree canopy is caused by natural regeneration (not planted)

o Soils in these areas are not likely to be compacted

o Enhanced biophysical processes in treed urban soils can contribute to stormwater reduction and denitrification

• Workshop participants thought that tree canopy should be a unique land use layer

• Credit is being given as a land use conversion for tree canopy expansion (in the Chesapeake Bay Watershed Model – CBWM)

o A panel will review this credit as to how much loading a tree canopy layer would receive

• An expert panel is being formed to evaluate urban tree canopy as an urban BMP

D. Sample, K. Berger, P. Claggett, J. Tribo, N. Goulet, B. Stack, S. Claggett, T. Schueler
Date Published
March 2015
Scientific and Technical Advisory Committee (STAC)
Publication Number
Stormwater Management, Infrastructure (green)
Modeling, Green infrastructure, XGI
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