Substrate moisture effect on water balance and thermal regime of a tropical extensive green roof

Green-roof thermal and hydrological performance has been extensively studied, but the specific effect of substrate moisture has received little attention. This study investigates the substrate moisture effect on evapotranspiration (ET), water balance and subaerial and subsurface temperatures of an extensive tropical green roof. Firstly, three weather types (sunny, cloudy and rainy) in conjunction with three substrate moisture states (wet, moist and dry) generate nine permutations for a scenario analysis. Secondly, the correlation analysis explores the relationship between substrate moisture and thermal performance indicators. The major finding is that substrate moisture is effective in regulating substrate thermal behavior, but less so in enhancing ET and associated cooling. Substrate moisture can notably cool the soil, rockwool and concrete tile on sunny days, and warm them on cloudy and rainy days. In contrast, substrate moisture has limited effect on ET, which is largely dependent on solar radiation, relative humidity and wind speed. The dry substrate on sunny day demonstrates an aberrant behavior of high ET which contradicts with previous studies. This unusual phenomenon is explained by the limited substrate mass effect of the thin extensive green roof. The vegetation surface and air temperatures show little variations between different soil moisture states, and their correlations with substrate moisture are insignificant. The findings could provide an additional substrate moisture dimension to enhance the design and management of green roofs with reference to water and thermal behavior.