Bioretention performance under different rainfall regimes in subtropical conditions: A case study in Sao Carlos, Brazil

Low Impact Development practices have emerged as alternative solutions for traditional urban drainage by restoring the pre-development hydrologic regime. In subtropical climate areas, the performance of these systems is still poorly understood. This study aims to assess the performance of a bioretention basin in a subtropical climate area during an entire hydrological year in order to analyze the differences between dry and rainy seasons. The main climatic factors and conditions influencing the runoff retention efficiency and peak attenuation were also analyzed in order to support bioretention design for flood control purposes. Data of 29 precipitation events were collected over three years (2016-2018). The results show that the bioretention system retained between 9% and 100% of the runoff volume with an average efficiency of 65% during a whole hydrological year. The average runoff retention efficiency was of 73% and 61% for dry and rainy seasons, respectively. This difference is explained by the climatic factors which affected the bioretention performance. During dry periods, the antecedent soil moisture condition and runoff generation rate were found to be more important than the total precipitation depth, while the runoff retention efficiency was primarily influenced by the total rainfall depth and the maximum rainfall intensity during the wet period. Future research should focus on each of these periods in more detail, including water quality aspects.