Heated surface runoff from impervious surfaces

Impervious surfaces absorb and store heat, which is then transmitted to surface runoff during rainfall events. Several studies have shown positive correlations between impervious surface area and stream temperature (Wang et al. 2003, Nelson & Palmer 2007, Imberger et al. 2008, Stranko et al. 2008).

Photo courtesy of U.S. EPA

Thompson et al. (2008a) compared runoff temperatures from asphalt and sod surfaces during 24 rainfall simulations (see Fig 28 for one of these simulations). They found that:

• Asphalt surfaces were more than 20°C warmer than sod surfaces prior to rainfall simulations.
• Initial asphalt runoff temperatures were roughly 10°C warmer than sod runoff temperatures (35.0 vs. 25.5°C).
• Asphalt runoff temperature decreased by an average of 4.1°C over the 1-hour rainfall simulation.

However, impervious surfaces do not always elevate stream temperatures. Many factors influence whether impervious surfaces generate heated surface runoff (Herb et al. 2008, Thompson et al. 2008b), including:

• Air temperature & humidity
• Type of impervious surface (e.g., reflectance)
• Solar radiation before & during rainfall
• Rainfall intensity
• Rainfall temperature

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