Effects of microfiber capillary wicking irrigation on soil moisture and evapotranspiration of green roofs

Abstract: Green roofs have drawn much more attention under the promotion of the Sponge City-Low Impact Development Rainwater System (LID) in modern urban construction and planning. Much more green roofs have been built so far. However, there is a great challenge on the later maintenance in the process of green roofs, such as the difficult irrigation, high cost, and large water consumption. Therefore, automatic irrigation is urgently needed suitable for roof greening. Taking the microfiber capillary as the material, this study aims to propose an automated irrigation technology for the roof green potted test, called the microfiber capillary wicking irrigation (MCWI). Taking the Antirrhinum majus, Murraya paniculata, Podocarpus macrophyllus, Portulaca grandiflora, succulent plant, and Sedum lineare as test plants, a systematic investigation was made to clarify the effects of MCWI treatment on the soil water content and evapotranspiration of green roof. The transpiration rate and evapotranspiration amount were measured for the different plants in different seasons using the specific steaming instrument. A three-temperature model was established to evaluate the MCWI performance on the evaporation of green roofs. The field experiment was carried out in the Zhuhai Campus, Sun Yat-sen University, Guangdong Province, China. The results showed that the average soil water contents of Murraya paniculata, Antirrhinum majus, and bare soil module with MCWI were 27%, 18%, and 47% higher than those without MCWI, respectively. The MCWI increased the soil water content, further reducing the occurrence of water stress. The transpiration rate of Antirrhinum majus with MCWI was significantly higher than that without MCWI (P>0.01) in summer (rainy season). The average daily transpiration rates of Antirrhinum majus were more than 13.8, 39.2, 45.7, and 19.0 J/(m2·s) on April 8, May 18, June 29, and July 27, 2021, respectively. Simultaneously, the maximum transpiration rates with MCWI were 22.3, 104.4, 94.0, and 30.8 J/(m2·s) higher than those without MCWI, respectively. The largest difference in transpiration rate between MCWI and non-MCWI was at 12:00-14:00, the hottest period of the day. Correspondingly, the higher the temperature was, the greater the temperature difference between plant canopy and air surrounding. By contrast, the bottom temperature of the test module with the MCWI was higher than that without MCWI in winter (dry season), no matter the day and night. The MCWI increased by 0.25 °C for the average temperature of the roof base, indicating a less temperature variation for the better energy storage of the green roof in winter. Evapotranspiration of Sedum line with MCWI at night was 0.14 mm higher than that without MCWI, but the MCWI evapotranspiration in the daytime was 0.57 mm lower than that without MCWI, indicating the overall decrease of 9.3% in the MCWI treatment. The change was attributed to the formation of dew in the process. Consequently, the MCWI can be used to improve the soil water content, the evapotranspiration of vegetation in summer, and to increase the roof evapotranspiration at night in winter. At the same time, it is also necessary to explore the air moisture for the green roof during the day, further to promote the non-precipitation resource. This finding can also provide sound theoretical support to the microfiber capillary wicking irrigation for water-saving green roofs.