Assessing short- and long-term modifications of steady-state water infiltration rate in an extensive Mediterranean green roof

Green roof detention capacity is related to the steady-state infiltration rate, i(s), of the growing medium. With the aim to investigate short- and long-term modifications of the detention capacity of an extensive Mediterranean green roof, three mini-disk infiltrometer (MDI) measurement campaigns were conducted at construction, after one season and after five years of operation. A laboratory experiment was designed to separately measure i(s) in the upper and the lower part of the substrate profile. During the first operating season, field i(s) increased by a factor of 2.4 and 1.9 for near-saturated (applied pressure head, h(0) = −30 mm) and quasi-saturated conditions (h(0) = −5 mm), respectively. Similar rainfall height did not induce significant modifications in the upper layer of the laboratory columns, even if contribution of small pores to water infiltration tended to increase. Differently, i(s) significantly decreased by a factor of 3.4–5.3 in the lower layer. After the simulated rainfall, the upper layer was less packed (mean bulk density, ρ(b) = 1.083 kg m(−3)) and the lower layer was more packed (ρ(b) = 1.218 kg m(−3)) as compared with the initial density (ρ(b) = 1.131 kg m(−3)) and the lower part enriched in small particles. Short-term modifications in the experimental plot were thus attributed to fine particles washing-off and bulk density decrease in the upper layer, yielding an overall more conductive porous medium. After five years of green roof operation, field i(s) did not further increase thus showing that the washing/clogging mechanism was complete after one season or it was masked by counteracting processes, like root development and hydrophobicity.