Session D10 - Evapotranspiration in SCMs

Evapotranspiration (ET) rates from Sustainable Drainage Systems (SuDS) differ significantly from reference ET rates due to differences in vegetation types, water availability and the urban setting. A technique capable of quantifying ET rates in situ would be of considerable value to urban drainage design and modelling professionals. In this paper we present and discuss findings from a continuous field monitoring exercise aimed at assessing whether the 3T method can deliver robust ET data in this context. The 3T method is based on the measurement of three temperatures: the surface temperatures of a transpiring leaf and an imitation leaf, alongside the air temperature. Estimates/measurements of the net radiation at both the vegetated and imitation leaf surfaces are also required. Results from a continuous monitoring study show a strong correlation between 3T-ET and ET losses determined directly using a weighing lysimeter. However, care must be taken to ensure that the measurements are made under valid atmospheric conditions, which may restrict time windows available for its deployment.

This study evaluated the combined effect of vegetation and internal water storage (IWS) on evapotranspiration (ET) dynamics in bioretention mesocosms. Twelve mesocosms were arranged under outdoor summer conditions in four configurations: planted/free-drainage (PF), planted/IWS (PI), unplanted/free-drainage (UF), and unplanted/IWS (UI). ET was determined by mass balance from periodic weighing, while soil moisture was monitored at three depths using sensors. Results showed that vegetation was the dominant control of ET, nearly doubling cumulative water loss compared to the unvegetated systems (~500 mm in PI compared with ~200 mm in UI). IWS further amplified ET by maintaining root-zone moisture, particularly during dry periods. Crop coefficients reached 3.0–3.5 in late summer for PI, demonstrating strong plant-driven recovery of retention capacity. The findings highlight IWS as an effective design enhancement for climate-resilient stormwater control.

Cities in Mediterranean climates face increasing water scarcity, heat stress and short, intense rainfall events. At the same time, large-scale adoption of blue–green roofs (BGRs) is limited by irrigation demands and the lack of robust, continuous performance monitoring. This study evaluates a water-efficient BGR configuration in which a roof-level reservoir supplies water to the substrate by passive capillary rise, with seasonal supplemental inputs designed to be comparable to on-site air-conditioner condensate volumes. An experimental roof at the Technion (Haifa, Israel) comprises two identical 100 m² plots: a reference plot with conventional surface drip irrigation and a test plot receiving water into the storage layer. High-resolution meteorological and hydrological observations are used within a two-layer (blue-green) water-balance framework to quantify evapotranspiration (ET), storage dynamics, internal fluxes and stormwater retention under constrained water supply. Sentinel-2 satellite imagery is processed to derive crop and stress coefficients (Kc, Ks), which are then used to estimate ET and support continuous roof-scale performance and water-stress monitoring. The findings are expected to advance practical, scalable approaches for designing and evaluating water-efficient BGR systems in drought-prone urban environments.