Climate-adaptive performance index for green roof substrates: year-round assessment in Auckland

Urbanisation and climate variability are increasing pressure on stormwater infrastructure, highlighting the need for robust performance evaluation of green roof systems. Conventional assessments often rely on single hydrological metrics, which may not capture temporal reliability and climate-dependent variability. This study develops a Climate-Adaptive Performance Index (CAPI), a multi-dimensional framework integrating runoff volume reduction, peak discharge reduction, performance consistency, and seasonal stability to comprehensively evaluate green roof substrates. The index was applied to four substrate configurations installed in seven experimental trays in Auckland, New Zealand, using high-resolution 3-minute rainfall–runoff monitoring over one year. Results showed strong stormwater mitigation, with runoff reduction exceeding 85% and peak discharge reduction exceeding 96% across substrates, while revealing differences in temporal reliability and seasonal adaptability among substrates with similar mean performance that were not evident from individual metrics. The CAPI framework provides a comparative, climate-responsive tool supporting evidence-based design and evaluation of green infrastructure systems.