Rock Creek Watershed Stormwater Runoff Analysis: i-Tree Hydro Report

This stormwater modeling case study was presented at the 17th Annual EPA Region 6 Stormwater Conference in Hot Springs, AR October 21, 2015.

Increasing tree canopy cover over a watershed mitigates stormwater runoff and pollution loading modestly. In the Rock Creek watershed, increasing tree cover by 30% (from 32% to 42%), we could expect total stormwater runoff to be reduced by about 2% and pollution loading by 2.5%. The results from this project show that the larger contributor to reducing pollution loading is by reducing the amount of directly connected impervious area and allowing more of the stormwater runoff to infiltrate into the soil before it reaches the stream.

Tree leaves and branches intercept rainfall and retain a portion thus preventing it from reaching the ground. Tree canopy also temporarily detains rainfall and releases it slowly as throughfall thus slowing the velocity of stormwater runoff. This slowing of stormwater velocity is beneficial for stormwater management as it allows soils and stormwater BMPs to work more efficiently and not become overwhelmed. By using trees in conjunction with reduced impervious area, we could expect to reduce pollution loading. In this project i-Tree Hydro estimated that pollution loading was reduced an additional 6.2% when tree canopy cover was increased and DCIA decreased by 30% compared to decreasing DCIA by 30% alone.

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