Moisture management in biochar-amended green roofs planted with Ophiopogon japonicus under different irrigation schemes: an integrated experimental and modeling approach

Green roof constantly suffers from the water stress that is developed during prolonged drought seasons. In general, periodical irrigation is required to ensure plant growth and hence serviceability of green roofs. Biochar, a carbon sink material, has been proposed as a substrate amendment in green roofs for enhancing water retention ability of soils. This study aims to conduct an assessment of the irrigation efficiency of green roofs with different biochar additions (0%, 5%, 10%, 15% and 20%; v/v) under sub-tropical climatic conditions. In order to achieve this objective, outdoor monitoring as well as numerical modeling using HYDRUS-1D was conducted. Soil columns mixed with different proportion of biochar were prepared. These columns were subjected to different irrigation schemes (three irrigation frequencies were assessed (i.e., per 3, 7 and 10 days after irrigation or rainfall); moreover, three irrigation amounts for the three irrigation frequencies were considered (i.e., to a fixed amount (FA_10mm), to Field water holding capacity (FC) and to Saturated moisture content (SR))). As suggested from the results: (1) Biochar significantly improved water holding capacity and plant available water. 20% biochar delayed the onset of the significant plant wilting phenomenon by approximately 3 days and maintained the maximal transpiration rate of vegetation in the dry period. (2) As compared to irrigation scheme A (irrigation to FC per 7 days), the efficiency of scheme B (irrigation to SR per 10 days) was more vulnerable to the biochar amendment. Moreover, the total irrigation water and days of water stress decreased with an increase in the biochar addition. Furthermore, the combination of 20% biochar and irrigation scheme B could be the optimal choice for maintaining the health of the green roofs and water conservation. The present study helps to obtain desired outcomes in green roofs, e.g., stormwater management, cost reduction as well as providing greening.