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CADDIS Volume 2: Sources, Stressors & Responses

energy sources physical habitat hydrology temperature water and sediment quality stormwater runoff wastewater inputs riparian and channel alteration urbanization

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

Courtesy of U.S. EPA.

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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Figure 17. Relationships between geometric means of baseflow (close circles, solid regression lines) and storm event (open circles, dashed regression lines) concentrations and two impervious cover variables: % drainage connection and % total imperviousness. R values provided as baseflow concentrations (storm event concentrations). DOC = dissolved organic carbon; EC = electrical conductivity; FRP = filterable reactive phosphorus; TP = total phosphorus; NH4+ = ammonium.
From Hatt BE et al. 2004. The influence of urban density and drainage infrastructure on the concentrations and loads of pollutants in small streams. Environmental Management 34(1):112-124. Reprinted with permission from Springer Scientific+Business Media.

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