The role of condensation from below in soil moisture dynamics within multilayer blue–green roofs

In urban areas, green roofs are increasingly adopted due to their multiple environmental benefits and their ability to mitigate hydro-meteorological risks. Among them, Multilayer Blue–Green Roofs include an additional storage layer that enhances pluvial flood mitigation by retaining excess water that percolates from the soil layer. This storage layer can also be regulated through a valve that allows controlled release of stored water into the urban drainage system. Existing hydrological models for blue–green roofs typically represent processes such as evapotranspiration, leakage and discharge, but the contribution of condensation from the underlying blue layer to soil-moisture dynamics is largely overlooked, despite monitoring evidence showing measurable moisture gains in the substrate associated with concurrent water loss from the storage layer. This study investigates the influence of condensation generated by upward water-vapor fluxes from the storage layer to the soil, assessing the impacts on the soil-moisture dynamics. The conceptual eco-hydrological model proposed by Viola et al. 2017 to simulate the soil-moisture dynamics of traditional green roofs, has been adapted to represents a Multilayer Blue-Green Roof, accounting for the additional storage layers and condensation dynamics. The Multilayer Blue–Green Roof prototype installed in the Engineering Faculty of the University of Cagliari has been selected as case study to calibrate the proposed model. The prototype has been equipped with sensors to continuously measure temperature, soil moisture, water level and discharge. Three years of collected data are available at high resolution for this Multilayer Blue–Green Roof. Rainfall and relative humidity data have been provided by the weather station network of the Regional Environmental Agency (ARPAS – Agenzia Regionale per la Protezione dell’Ambiente Sardegna). Crop coefficient and mass transfer coefficient have been calibrated for each season with the aim to account for the different vegetation cover. Incorporating condensation processes significantly improved model performance, yielding soil-moisture and water-balance simulations closely aligned with observations. Results highlight condensation as a non-negligible process in Multilayer Blue–Green Roofs hydrology and support its inclusion in future roof modelling frameworks.