Green roofs: effects of hydrogel on hydrological and ecological behavior

Green roofs are frequently exposed to extremely hard condition, in particular in Mediterranean area due to the shallow depth and low water holding capacity of the substrate, and to the custom of reducing maintenance costs, avoiding irrigation. This study presents the effects of a hydrogel polymer on the hydrological behavior and on growth of Sedum hispanicum L. and Bromus erectus Huds. Plants were grown in six test beds containing 10 cm of substrate mixed with three concentration of hydrolgel: 0% w/w (2 modules with substrate itself), 0.3% w/w (2 modules) and 0.6% w/w (2 modules). Weather data were collected continuously, while runoff was measured for three events. The water losses, due to evapotranspiration (ET, in mm/day), were evaluated by measuring the difference between the daily changes in weight of the boxes. Furthermore, two plant traits (leaf dry matter content - LDMC and relative water content - RWC) were used to evaluate the ecological behavior of the species. RWC was measured during the runoff measurement; LDMC was measured at the beginning of the experiments and after 3 months. Preliminary results show that the presence of hydrogel strongly influence the retention performance of green roofs, in fact the higher the hydrogel concentration the higher the retention. Hydrogel increases the substrate water content at saturation, as well as the water available for vegetation and consequently the greater the hydrogel concentration and the greater the daily losses for ET. Regarding the ecological performance, the preliminary results show that the presence of hydrogel does not influence the species RWC, moreover the LDMC values show a decreasing after 3 months especially for Bromus. LDMC has been shown to correlate negatively with potential relative growth rate; the presence of hydrogel then helps the vegetation growth.