Predicting Ecological Effects of Watershed-Wide Rain Garden Implementation Using a Low-Cost Methodology

Watershed Protection

Predicting Ecological Effects of Watershed-Wide Rain Garden Implementation Using a Low-Cost Methodology

This paper highlights many of the challenges that the Foundation and partner organizations face in our efforts to achieve measurable improvements to water quality by reducing the stormwater pollution that runs off our developed lands.  As has been shown in many studies, even small amounts of impervious landcover within a watershed impair water quality and watershed health. Strategies to address this problem frequently include physical installations designed to soak up the stormwater associated with impervious surfaces and prevent it from impacting nearby streams. However, while stormwater installations and ongoing monitoring are both fundamental aspects of the Foundation’s Watershed Protection strategy, there are significant challenges in both estimating and detecting the positive influence of such installations on the waterways they are designed protect.

In this study, led by researchers that include WPF grantee Villanova University, we are asked to consider a new assessment method that examines the effects of stormwater installations, namely rain gardens, on ecological indicators of stream function (i.e., habitat condition) as well as the more easily and more frequently modeled hydrological indicators (e.g., flow, volume); and perhaps in place of more expensive biological indicators like species health and composition. From their assessment we learn that an increase in impervious cover from 3% to 5% of an 8 square mile sub-watershed can be expected to reduce available habitat for an important fish species by 14%, while impervious cover over 25% eliminates that habitat entirely. Furthermore, in scenarios where the sub-watershed has impervious cover between 5% and 25% the installation of rain gardens can typically be expected to exert a small but significant improvement in available habitat; but at 60% impervious cover, conditions are simply too degraded to affect with rain gardens.

Near where this study was based in the Upstream Suburban Philadelphia sub-watershed cluster of the Delaware River Watershed Initiative, the average impervious cover is well above the threshold for improvement at 67%. Accordingly, findings such as these have important implications for how conservation practitioners scale their approaches, whether to the 8 square-mile sub-watershed or the 2 square-mile “micro”-watershed; and how they select from available interventions, whether they be rain gardens or high capacity detention basins, etc. With approaches and interventions in turn relating back to specific and measureable watershed protection goals, we should be able to formulate specific hypotheses for positive impact and increasingly make use of the rich and growing body of research that is emerging from this work.

This study was funded in part by the William Penn Foundation under our Watershed Protection strategy.

Published: August 2014
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