COS 107-3
Stormwater performance of a full scale rooftop farm
Conventional green roofs provide a wide variety of benefits including reduction of stormwater runoff and improvement of runoff quality, which are the basis for a number of incentive programs. It is, however, unclear how agricultural production on green roofs affects these benefits and eligibility for such incentives. This could shape the spread of rooftop agriculture and its role in sustainable cities of the future. In order to examine the stormwater impacts of rooftop farming, monitoring equipment was installed on the Brooklyn Grange, a full scale rooftop farm in Long Island City, Queens, New York. Rain and runoff water samples were collected and analyzed for pH, electrical conductivity, turbidity, and apparent and true color Heffner Laboratory at Columbia University. A portion of the sample was also sent to Auburn University Soil Testing Laboratory (Auburn University, AL) for analysis for routine elements by ICAP and ammonium-N. A weather station was used to monitor ambient weather conditions and v-notch weirs fitted with ultrasonic distance sensors were used to monitor the amount of water running off the roof from one drainage basin. Results were then compared to those of two previous studies performed on extensive sedum green roofs, non-vegetated roofs and rain in New York City.
Results/Conclusions
Runoff water from the Brooklyn Grange tested higher than that of extensive sedum green roofs for conductivity, apparent color, nitrate-nitrogen, phosphorus, potassium, calcium, magnesium, and sodium, likely because of higher fertilizer use and organic matter content of the green roof substrate. The Brooklyn Grange green roof does still have a neutralizing effect on the pH of water compared to that of rain, if not to the same extent as extensive sedum green roofs in previous studies. Further research on the effect of nutrient management practices on runoff water quality would provide recommendations that could improve runoff water quality. The Brooklyn Grange was found to retain 40% of rain, on the low side for extensive sedum green roofs. It also retained less runoff as the amount of precipitation in individual storms increased than an extensive sedum green roof. The use of irrigation on the Brooklyn Grange rooftop is not typical of extensive sedum green roofs and the likely cause of the roofs lower stormwater retention. Comparison of the moisture content on the Brooklyn Grange and an extensive sedum green roof support this conclusion.