Baseflow in urban streams

Urbanization generally results in increased magnitude and frequency of peak flows, but baseflow effects typically are more variable, with studies showing a range of responses in urban streams [Lerner 2002, Brandes et al. 2005, Meyer 2005, Roy et al. 2005 (Fig 35), Poff et al. 2006].

Decreases in baseflow may result from:

• ↓ infiltration due to ↑ impervious surfaces
• ↑ water withdrawals (surface or ground)

These decreases may be offset, however, by increases in baseflow resulting from:

• ↑ imported water supplies (i.e., interbasin transfers)
• ↑ leakage from sewers and septic systems
• ↑ leakage from water supply infrastructure
• ↑ irrigation (e.g., lawn watering)
• ↑ discharge of wastewater effluents
• ↑ infiltration due to water collection in recharge areas
• ↓ evapotranspiration due to ↓ vegetative cover

Urban-related increases in baseflow can be especially evident in effluent-dominated systems, or streams and rivers in which wastewater effluents comprise a significant portion of baseflow volumes. For example:

• Discharge from two wastewater treatment plants accounted for at least 70% of river flow in the Bush River, SC in Summer 2002 (Andersen et al. 2004).
• Average effluent flow in the South Platte River, CO is 41% total streamflow; during low flow conditions, this can increase to 90% (Woodling et al. 2006).

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