Effect of Three Vegetations on Rainwater Retention in Extensive Green Roofs: A Comparative Study

Abstract

A green roof is a building innovation that plays a role in sustainable rainwater management.
This research aimed to observe the effects of three vegetation (C3, C4, and CAM), which had different photosynthesis processes on rainwater retention efficiency. The operation experiment was conducted by installing four experiment boxes: a concrete roof box and a green roof test box planted with C3 (Arachis pintoi Krapov.&W.C.Greg.), C4 (Axonopus compressus (Sw.) P.Beauv.), and CAM (Callisia repens.) to compare the rainwater retention efficiency. Using methods to collect rainwater data in Bangkok from July to October and transform data to simulate rain events and antecedent dry weather periods (ADWP), respectively. Simulated rain events were conducted as following; Event 1: moderate rain event: 22 min with ADWP 2 days, Event 2: heavy rain event: 100 min with ADWP 3 days, and Event 3: moderate rain event: 26 min with ADWP 4 days.

Findings illustrated the water use efficiency of vegetation was the significant factor affecting the rainwater retention efficiency of green roofs. C4 had excellent rainwater retention efficiency, followed by C3 and CAM. In Event 1, C4 performed the best retention, followed by C3, and CAM (77-82%, 45-60%, and 27-28%), respectively. In Event 2, C4 and C3 retained 9-11% and 1-6% of rainwater, while CAM did not retain rainwater. In addition, in Event 3, C4 still showed the highest retention capacity, followed by CAM, and C3 (70-73%, 57-64%, and 40-54%), respectively. In addition, measured weather data during experiments (Rainfall intensity, antecedent dry weather periods: ADWP, and weather data) contributed significant factors in plants transpiration. As a result, a C4 plant, which has the highest transpiration rate and there is the highest requirement for water in the growth process as well, performed the best rainwater retention. Furthermore, the physical characteristics of C4 as a ground cover crop with high density and large leaves brought better water interception ability than C3 and CAM. Consequently, C4 (Axonopus compressus (Sw.) P.Beauv.) was the most efficient vegetation for rainwater retention. However, a sufficient irrigation system and maintenance strategies should be designed before application.