The Role of the Residential Urban Forest in Regulating Throughfall

By Elina N.M. Inkiläinen, Melissa R. McHale, Gary B. Blank, April L. James, and Eero Nikinmaa

Raleigh, NC (November 2013) – New research carried out in 16 residential yards in Raleigh, NC found that urban residential forests reduced potential stormwater runoff by 9.1–21.4%.  Canopy cover (p < 0.0001) and coniferous trees (p = 0.017) were the most influential vegetation variables explaining throughfall.

Crepe_myrtle_charleston_scOverwhelming stormwater volumes, associated with deteriorating water quality and severe flooding in urbanizing cities, have become a great environmental and financial concern globally. Urban forests are capable of reducing the amount of stormwater runoff, in part, by regulating throughfall via canopy rainfall interception.

The lack of stand-scale studies of urban throughfall, however, hinders realistic estimates of the benefits of urban vegetation for stormwater regulation. Furthermore, urban forest characteristics that may be influencing rainfall interception are difficult to establish as these environments are extremely heterogeneous and managed, to a large extent, by private residents with varying landscape preferences.

To quantify the amount of rainfall interception by vegetation in a residential urban forest researchers measured throughfall in Raleigh, NC, between July and November 2010. They analyzed 16 residential yards with varying vegetation structure to evaluate the relative importance of different descriptive measures of vegetation in influencing throughfall in an urban watershed.

Research highlights:

  • Urban residential forest reduced potential stormwater runoff by 9.1–21.4%.
  • Throughfall amounts ranged greatly among yards with 41.5–88.3% canopy cover.
  • Variability found in vegetation and throughfall between private and rented yards.
  • Differences also found between front and backyards.
  • Canopy cover (p < 0.0001) and coniferous trees (p = 0.017) influential variables.

Throughfall comprised 78.1–88.9% of gross precipitation, indicating 9.1–21.4% rainfall interception. Canopy cover (p < 0.0001) and coniferous trees (p = 0.017) were the most influential vegetation variables explaining throughfall whereas variables such as leaf area index were not found significant in our models.

Throughfall and vegetation characteristics varied significantly among yards (p < 0.0001), between front and back yards (p < 0.0001), and between rented and privately-owned yards (p = 0.001), suggesting a potentially significant role in stormwater regulation for urban residents.

Source: The role of the residential urban forest in regulating throughfall: A case study in Raleigh, North Carolina, USA,” Landscape and Urban Planning, November 2013.