Study on rainwater retention effect of an extensive green roof

Green roof is proved to be one of the best urban rainwater management means. This study is to evaluate the effect of an extensive green roof on rainwater retention and runoff process during individual rain storms under the climate of Central China. In situ tests was conducted on the simulated green roof plot together with a simulated conventional roof plot on a roof top of a Teaching building in Xiangfan University to compare minute runoff quantity during two rain events and rainwater retention from the two plots in the year of 2009. The results show that compared with conventional roof, the extensive green roof can reduce runoff by retaining more rainwater whether in an individual rain event, or in a month, or in a season, or in the whole year; and the green roof retained 176.4 mm more rainwater than the conventional roof, which is 23% of annual precipitation in 2009. Rainfall volume and the substrate moisture content play an import effect on rainwater retention. The green roof has stronger retention effect on lighter rainfalls than heavier ones, and provides very limited retention for extra heavy rain storms. And the green roof can retain rainwater more efficiently if the substrate layer is dry. Another factor affecting water retention is the duration of a rain event. The green roof (or conventional roof) can retain more water if the rain fall duration prolongs. And the runoff processes from the two plots under two rain storms shown that compared with conventional roof, the extensive green roof can significantly detain rainwater, delay the initial time of runoff , lower the peak flow of runoff and the runoff volume, even when the substrate layer is wet from previously rain fall.