Hydrology of the Brooklyn Grange, an urban rooftop farm

Intensive agriculture is an emerging theme of green roof technology. Production of affordable, nutritious vegetables for local consumption and stormwater management are among the ecosystem services emphasized in studies of rooftop farming. However, intensive vegetable production requires irrigation, which can reduce stormwater retention, and there is little empirical data from full-scale operational rooftop farms. In this paper, we report the hydrologic performance of the Brooklyn Grange Navy Yard Farm, a 0.61-ha rooftop farm atop an 11-story building in New York City USA. We monitored soil water content, precipitation, irrigation, drainage, and crop yields to provide water balance and water use efficiency. We found cumulative discharge exceeded precipitation by 11% during the entire study period, hence the farm was a net source of water in the urban hydrologic cycle. Depending on crop types, water use efficiency at the Grange was lower than in-ground agriculture in dry regions with high irrigation demands. For the Brooklyn Grange to integrate stormwater management and water-efficient crop production, it will be important to increase soil water storage, to reduce irrigation demands and drainage loss of water while maintaining average soil moisture levels in the range of readily available water between 10 and 100 kPa of soil water tension.