Date of this Version
Schwarz, C. (2015). Green roof energy balances for native grass and sedum.
Green roofs offer a possible solution to reduce urban heat island effects caused by urbanization. Conventional green roofs use sedum for plant cover but growing evidence suggests native species offer better results for harsher environments. However, more information is needed to understand the potential of native species for green roof use in terms of survivability and cooling potential. This study was conducted from July 2013 through October 2013 to quantify the energy balance components for native grasses and sedum on a Midwestern green roof. The semi-intensive green roof located on the Larson Building parking garage in Lincoln, NE was used for this study. Two areas, one sedum with a medium depth of 10 cm and the other of native grasses with a medium depth ranging between 13 cm and 15 cm, were investigated using instrumentation to record solar radiation, thermal energy, and mass fluxes, in addition to other micrometeorological conditions. During the study period daily average net radiation (Rn) and daily average available energy (Rn-Gsfc, where Gsfc is the daily soil heat flux) were similar in the native grass and sedum areas. The native grass area had the higher daily average latent heat flux (LET) and daily average albedo with 9.0 MJ m–2 d–1 and 0.18, respectively. This finding is in part due to the greater depth of growth medium in the native grass and the partial vegetative cover in the sedum. Sedum was found to have the higher daily average sensible heat flux (H) and daily average soil heat flux (Gsfc) with 4.9 MJ m–2 d–1 and 0.88 MJ m–2 d–1, respectively. The native grass area was found to have significantly higher daily LET, significantly lower Gsfc and H, and higher percentages of vegetative cover over the growth medium. These traits and conditions led the native grass to outperform the sedum in areas of survivability and cooling capacity. This result provides further evidence that native species are an appropriate choice for green roof use.
Advisors: Kenneth Hubbard, Elizabeth Walter-Shea, and Richard Sutton