Accounting for Trees in Stormwater Models

This paper is intended to help the stormwater engineering community more easily account for trees in runoff and pollutant load calculations so that they can more readily incorporate them into their stormwater management strategies.

Site designers and stormwater engineers who incorporate trees into their projects at the site and catchment scales can reduce runoff and therefore reduce the costs associated with structural practices needed to treat site runoff or reduce pollutant loads. However, to realize this cost savings, the runoff reduction associated with trees must be accounted for in stormwater runoff computations. Unfortunately, the necessary data and models to conduct such analyses have not been readily available to the stormwater community. This has hampered the ability to take regulatory credit for tree planting and tree preservation efforts in stormwater permits, long-term CSO plans/consent decrees or TMDL implementation plans.

Due to recent advances in our understanding of the stormwater benefits of trees, some modeling programs have been updated to account for these benefits. Other models, while not explicitly updated to quantify tree benefits, can still be used to account for them by modifying specific model inputs. The purpose of this fact sheet is to summarize this information for site designers and stormwater engineers so that they can more easily account for trees in their runoff and pollutant loading reduction computations. The fact sheet focuses on existing hydrologic and hydraulic models and other tools that can be applied at the site, catchment and watershed scales to account for the stormwater benefits of conserving existing trees and/or planting new trees. With this information, trees can more readily be used for stormwater management purposes.

Accounting for Trees in Stormwater Models