Green roof evapotranspiration rates and stormwater control under subtropical climate: a case study in Brazil

ABSTRACT In this study a long-term field experiment evaluating evapotranspiration rates from irrigated and non-irrigated green roof modules, as well their impacts on stormwater control was accomplished. Six green roof modules (3 irrigated and 3 non-irrigated) vegetated with S. rupestre were monitored throughout 8 months in southern Brazil. Four non-vegetated modules (2 irrigated and 2 non-irrigated) were simultaneously assessed to understand the role of the vegetation in the whole process. The average evapotranspiration under water-stress (ETr) was 2.6 mm.day-1, while mean evapotranspiration under water-abundance (ETp) was 2.8 mm.day-1. Higher evapotranspiration rates were observed during summer, increasing the substrate storage capacity, although ETr amount along the seasons was very similar, mainly affected by climatic conditions. The long-term analysis showed that 47% of the total rainfall was converted into runoff, 21% was retained in the green roof modules and 32% was released through evapotranspiration, reinforcing the importance of vegetation as a mechanism for improving stormwater control benefits. The results of this research also allowed the establishment of a crop coefficient (Kc) time series, with a monthly average of 0.9 which permits the S. rupestre evapotranspiration to be preliminarily estimated by using equations developed for reference culture without the need of monitoring. RESUMO Neste estudo foi realizado um experimento de campo de longo prazo avaliando as taxas de evapotranspiração de módulos de telhados verdes irrigados e não irrigados, bem como seus impactos no controle de águas pluviais. Seis módulos de telhado verde (3 irrigados e 3 não irrigados) com vegetação de S. rupestre foram monitorados ao longo de 8 meses no sul do Brasil. Além disso, quatro módulos sem vegetação (2 irrigados e 2 não irrigados) foram avaliados simultaneamente para compreender o papel da vegetação em todo o processo. A evapotranspiração média sob condição de estresse hídrico (ETr) foi de 2,6 mm.dia-1, enquanto a evapotranspiração média sob condição de abundância hídrica (ETp) foi de 2,8 mm.dia-1. Maiores taxas de evapotranspiração foram observadas durante o verão, aumentando a capacidade de armazenamento do substrato, embora a quantidade de ETr ao longo das estações tenha sido muito semelhante, afetada principalmente pelas condições climáticas. A análise de longo prazo mostrou que 47% do total da chuva foi convertido em escoamento, 21% foi retido nos módulo