The Effects of Biochar on Hydrologic and Thermal Performance of Green Roof Substrate Having Water Retention Layer.

Abstract

Among goals of green roof technology are stormwater management and cooling in summer, which can be improved by using the lower part of the substrate layer for water storage, while few studies tested the effects of mixing biochar to green roof substrates having such water storage system. In this study, the small (< 0.25 mm) vs. large (0.25-2.00 mm) particle size biochar（0, 50, 100, 200 t/ha）was mixed with the 20 cm deep green roof substrate layers of pots, having drainage holes 10 cm above the bottom allowing the bottom half of the substrate layer to store water. In the 28-day laboratory experiment, hydrological and thermal properties were measured by recreating heavy rainfall and the summertime urban heat island effect (60℃) of Tokyo, Japan by using the high-output LED. We found that the green roof substrate mixed with the 200 t/ha small particle biochar achieved the highest field capacity (44.7%) and plant available water (35.5%), while the water retention rates tended to decrease for green roof substrates mixed with 200 t/ha large particle biochar. The addition of the 100-200 t/ha large particle biochar increased the substrate surface temperature for 6.5-12.6℃, while substrate temperature 5 cm below the surface was not different among samples. Further research is needed for understanding the hydrothermal effects of vegetation on the biochar-amended green roof substrate layers having the water storage system.