A HYDRUS model for irrigation management of green roofs with a water storage layer

• The effect of evaporation from the storage layer on the soil moisture was simulated.
• The efficiency of various irrigation schemes for the green roofs was evaluated.
• Without irrigation the green roofs experience water stress in >35% of days in a year.
• Irrigation to field capacity every 3 days is a relatively efficient irrigation scheme.
• Increasing storage layer depth will decrease both water stress and irrigation amount.

Green roofs suffer from water stress during prolonged dry periods. Periodically irrigation is usually needed to maintain the health of green roofs. This paper presents a green roof model for simulating the long term variation of soil moisture of green roofs with a storage layer. The model mainly incorporates a HYDRUS-1D model for the soil layer and an evaporation model for the storage layer. It explicitly accounts for the effects of evaporation from the storage layer on the soil moisture. After calibration and validation with the observed data of a pilot green roof, the model was implemented to evaluate the efficiency of various irrigation schemes for the green roofs built with different structural designs under typical weather conditions in Shenzhen, China. The results indicate that without irrigation the green roofs experience water stress (defined as soil moisture <60% of the field capacity) in more than 35% of days in a year. As the irrigation frequency increases, the days of water stress decrease but the total irrigation amount increases. Irrigation to the field capacity every 3 days and irrigation to the saturation moisture content every 7 days are two relatively efficient irrigation schemes, which can achieve optimal tradeoffs between the objectives of saving irrigation water and maintaining adequate soil water availability to the green roof plants. Furthermore, as the soil layer depth increases, the days of water stress decrease but the total irrigation amount increases. While as the storage layer depth increases, both the days of water stress and the total irrigation amount decrease. The study demonstrates that the model has the capacity to evaluating the efficiency of the irrigation schemes for the green roof. Therefore, it can help support the irrigation management of green roofs.