Observations of water vapor, heat and CO2 exchanges over a living roof using eddy covariance

Roofs cover approximately one third of the planimetric surface area of cities, which makes them a significant driver of the urban boundary layer. Conventional roofs act along with other impervious urban surfaces to greatly alter the surface energy, water and carbon budgets relative to the natural vegetation and soil surfaces they replaced. Living roofs therefore have been recommended to help limit urban hydroclimate modification due to several biophysical controls including, atmospheric carbon uptake and storage in soil and plant matter, building energy conservation via evapotranspiration and increased insulation and reduction in runoff. Micrometeorological measurements including the eddy covariance approach were used to estimate CO₂, water vapor and heat fluxes on the California Academy of Sciences living roof in Golden Gate Park, San Francisco, California. The roof's area is 18,000 m², containing 1.7 million plants from nine native Californian species and is the largest living roof in the state. Measurements were made about 10 m inside the eastern edge of the roof, at one meter above the 10-20 cm tall vegetation in order to capture adequate rooftop fetch for the prevailing summertime northwesterly flow. Following data reduction due to fetch limitations and low friction velocity (below 0.2 m s^-1), diurnal summertime patterns of CO₂, water vapor and heat are estimated. On a daily basis, the roof acted as a carbon sink of approximately 1.5 gC m^-2 d^-1 for a roof total of 2.75 kgC d-1. Turbulent heat fluxes were dominated by sensible heat with a mean Bowen ratio of approximately 1.5 and daily evapotranspiration rates of about 1.8 mm d^-1. These values are similar to those found for dry summer mediterranean shrubs elsewhere.