Green Water from Green Roofs—The Ecological and Economic Effects

Agnieszka Bus,Anna Szelągowska

doi:10.3390/su13042403

Abstract

Green roofs (GRs) have been one of the most popular solutions for water harvesting in urban areas. Apart from their water retention role and increasing biodiversity, they constitute the missing link between the built and the natural environment, which is required for sustainable human living in cities. This paper aims to calculate the ecological (EE) and economic effect (EcE) of water harvesting via GRs, by contrasting with a traditional roof, and to perform an economic analysis of the social cost benefits that GRs generate during their life cycle, using the Net Present Value (NPV) method. All the calculations and analyses were conducted for both intensive and extensive GRs in 11 of the largest municipalities in Poland, with a population of >250,000 inhabitants. According to the results of this study, water retention and the economic and ecological effects of GRs are highest in the municipalities with the highest assumed number of GRs (Warsaw, Krakow, Wroclaw, and Szczecin). The average EE and EcE equals 507,000 m3/yr and 621,000 USD/yr. The NPV results show that the effectiveness of investments in intensive GRs is, to a certain extent, more significant than in extensive GRs and the average equals 60.77 and 4.47 USD/yr for intensive and extensive GRs, respectively. The results could serve as a reference for the evaluation and optimization of the energy efficiency of rainwater harvesting schemes, in European cities.

Highlights

The EE in terms of water retention is calculated as the difference between the drained water on the traditional roof and the water retained on the Green roofs (GRs)
The water retained on GRs is calculated by multiplying the GR area in the city, precipitation, and the index of GR water retention from GRs (60%)
According to the results of this study, the EE and economic effect (EcE) of GR water retention are most significant in cities with the largest GR area (Warsaw, Krakow, Wroclaw, and Szczecin)

Summary

Introduction

Urban areas are becoming a living environment for an increasing number of people, and more than 70.9% of the total EU-28 population lives in cities, towns, and suburbs [1]. It is estimated that by 2050, more than 80% of the EU-28 population will live in cities [1,2] and further that the estimation of Europe’s level of urbanization is expected to increase to approximately 83.7% in 2050 [3]. Cities are major socioeconomic centres in which more than 50% of the global population live and where the population is forecast to double by 2050. These are considerable challenges in terms of maintaining and restoring green spaces within urban areas. Green roofs (GRs) ( known as living roofs, vegetable roofs, or eco-roofs [5])

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