CADDIS Volume 2: Sources, Stressors & Responses
Effective vs. total imperviousness
The effects of urbanization on stream ecosystems are largely driven by impervious cover. There are two general ways to quantify impervious cover:
Total impervious area (TIA) = all impervious area in catchment
Effective impervious area (EIA) = impervious area in catchment that is directly connected to stream channels (i.e., precipitation falling on that area is effectively transported to the stream)
Several methods can be used to determine EIA, with varying levels of accuracy (Roy & Shuster 2009). They include:
- Geographic information system data combined with overlays of stormwater infrastructure
- Published empirical relationships between TIA and EIA (Alley & Veenhuis 1983, Wenger et al. 2008)
- Field assessments
Many studies have found that EIA (also known as drainage connection or directly connected impervious area) is a better predictor of ecosystem alteration in urban streams. For example, Hatt et al. (2004) showed that % connection was more strongly related to water chemistry variables (e.g., conductivity, total phosphorus) than % total imperviousness, during both baseflows and stormflows (Fig 17).
The strength of EIA relationships suggests that stormwater management techniques aimed at disconnecting impervious areas from stream channels can improve urban water quality (Walsh et al. 2005b).
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