The impact of different vegetation densities and biochar particle sizes in substrate layers on rainfall infiltration process in green roofs under rainstorm intensity

Green roofs are widely used for urban stormwater management, yet the combined effects of vegetation density and biochar particle size on substrate moisture dynamics under storm-intensity rainfall remain unclear. This study evaluated four vegetation densities (0, 1/3, 2/3, and 3/3) and four biochar particle-size treatments (0, 1–3, 3–5, and 5–8 mm) in extensive green roof substrates. Sixteen treatment combinations were tested under simulated rainfall of 100 mm h -1 for 60 min, with three independently prepared modules per treatment. Increasing vegetation density generally shortened the initial moisture-response time and time to peak moisture content, while reducing peak and final moisture contents. This pattern may be associated with root-induced preferential flow after canopy storage was rapidly exceeded. Biochar particle size produced contrasting hydraulic responses. The 1–3 mm treatment showed the highest peak and final moisture contents and the latest response, indicating stronger temporary water retention. In contrast, the 5–8 mm treatment responded earlier and retained less water, indicating faster drainage. These results show that vegetation density and biochar particle size jointly regulate the retention–drainage balance. Under the tested conditions, finer biochar favoured temporary water storage, whereas coarser biochar and higher vegetation density promoted faster drainage.