A method to analyze the food-water-energy nexus for data-sparse cities: A comparison of green roofs in São José dos Campos, Brazil and Johannesburg, South Africa

Abstract

Green roofs have gained relevance in urban sustainability and affect the urban food-water-energy nexus in cities in many different ways. However, it remains unknown regarding systemically quantifying the trade-offs of green roofs on urban food-water-energy nexus. Most city-scale research is restricted to a silo lens (e.g., life cycle impacts and food-water-energy related benefits) while research focusing on green roof’s multiple trade-offs occurs at smaller scales (e.g., neighborhood). Here we developed a generalizable integrated methodology framework for data-sparse cities to analyze the food-water-energy nexus of green roofs from an urban system perspective and tested the framework in two global south cities, São José dos Campos, Brazil and Johannesburg, South Africa. We find São José dos Campos could reach its total self-sufficiency of vegetables in prospect by green roof farming, whereas its life cycle energy consumption would achieve 45% of urban energy demand in the worst scenario. Fueled by the scheme of rainwater substitution, 96% of the life cycle water consumption in São José dos Campos rushed to the infrastructure stage from the operation stage. Johannesburg had potential to satisfy 73% of urban vegetable self-sufficiency and contribute 8% to the city’s alternative water supply goal by rainwater harvesting. Local food supplies are cardinal drivers of trans-boundary water footprints beyond the cities of both São José dos Campos and Johannesburg. We highlight the necessity to balance the trade-offs of green roofs orienting urban food-water-energy goals and call for opportunities for green roof practice in cities.

The food-water-energy nexus and green roofs in Sao Jose dos Campos, Brazil, and Johannesburg, South Africa

“…1), this study shaped the dynamic links and trade-offs between green roofs and FWE nexus. The method descriptions are as follows (see Supplementary Methods for more details), and the raw data and calculation processes are available in the Supplementary data 55 .…”

“…(2) soil data, including the land use type and top soil texture; (3) cropping data, including the crop parameters, fertilization, and irrigation (see data 55 for the parameters).…”

“…This study selected two proxy American cities in the same climate zones as SJC and Johannesburg based on the ASHRAE climate zone (Supplementary Table 5). Based on the input values for building type, location, leaf area index of green roofs, growing media depth, and irrigation operations (see Supplementary Methods 2 and Supplementary data 55 for the details), the simulation results could be interpolated.…”

“…This study assumed that the growing period for tomato crops on green roofs in the case cities was 150 days 60 , and there could be two growth cycles each year. Thereby, the annual carbon capture per unit area of the studied green roofs could be estimated (see Supplementary data 55 for the details).…”

“…Here it was assumed that the entire food, water, and energy resources of SJC and Johannesburg were provided from other regions within the national territory. Therefore, this study constructed Brazil-and South Africa-based EIO-LCA models based on the GTAP v10 database, and additionally used the current local prices of products (i.e., water, electricity, gas, and tomato) and consumer price index (CPI) to convert the physical products into economic values for each sector (i.e., food, water, and energy sectors) (see Supplementary Methods 3 and Supplementary data 55 for the details).…”