Green roofs and rain gardens can’t tell engineers how much water they’ll hold between storms, so drainage systems are often overbuilt or underbuilt. This project will measure how much water vegetated SuDS lose to the air through evapotranspiration—the combined process of evaporation from soil and water release from plant leaves—so designers can predict retention capacity with confidence. Without reliable evapotranspiration data, engineers either oversize systems, wasting money and space, or undersize them, risking floods and sewer overflows into rivers. Current models use generic crop estimates that ignore the complex canopies, shading, and urban microclimates of real SuDS. The team will use thermal cameras on drones and a sensing vehicle to measure actual evapotranspiration across test beds, individual devices, and a full green infrastructure corridor in Sheffield. They will validate those remote readings against direct measurements of rainfall, runoff, and soil moisture. If successful, the research will produce practical design guidance that lets engineers specify SuDS with known, reliable retention capacities. That could reduce flood risk, cut combined sewer overflows, and make cities more resilient to climate change—without guesswork.
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Evapotranspiration (ET) represents a key flux in urban heat & water budgets, cooling cities, mitigating flood risk and reducing Combined Sewer Overflow (CSO) discharges into urban watercourses. The role of ET in the 'urban metabolism' makes it a key component in our efforts to deliver climate change resilient cities. The stormwater management benefits associated with ET can be significantly enhanced through retrofitting vegetated Sustainable Drainage Systems (SuDS) such as green roofs, bioretention cells and rain gardens. In the days between storm events, the removal of soil moisture from SuDS generates capacity for rainfall-runoff retention. However, knowing how much retention to expect represents a significant challenge for SuDS designers. This uncertainty around retention capacities for SuDS may lead to under- or over-design, or prevent their implementation altogether. Continuous simulation modelling of SuDS allows drainage engineers to understand the dynamic wetting and drying cycles that occur in response to rainfall and ET respectively. Model results can provide probabilistic estimates of the expected retention capacity; a designer could utilise either a median expected retention depth (50th percentile) or a more conservative estimate (e.g. the 90th percentile), depending on risk attitude and regulatory requirements. However, robust model outputs are dependent upon credible estimates of rainfall and SuDS-specific ET rates. Whilst global climate models provide estimates of expected future rainfall time-series, we do not yet have reliable information regarding ET rates for SuDS. Potential ET can be estimated from climate data for a standard reference crop. However, Actual ET rates associated with SuDS vegetation may be significantly different depending on the plant characteristics and the availability of moisture. Bioretention cells are deliberately planted with multiple diverse species, providing year-round colour/interest and maximising the landscape value and biodiversity benefits. Structural diversity means there may be multiple overlapping plant canopies and shading effects, with complex implications for overall ET. ET rates for SuDS vegetation will also be affected by fragmentation and/or massing of vegetation, and urban microclimate effects such as street canyons. This complexity necessitates new research to quantify ET for vegetated SuDS. We will utilise multiple techniques to quantify the Actual ET rates for SuDS vegetation across experimental scales ranging from controlled test beds to specific SuDS devices, and finally a corridor of extensive SuDS Green Infrastructure in the City of Sheffield. We will develop and apply a non-invasive ET quantification techniques in the field based on thermal remote sensing. Whilst these techniques are utilised extensively in the agricultural and natural sciences, their use here, to characterise the functionality of an engineered urban drainage device, is novel and adventurous. The techniques will be refined and validated against direct measurements of rainfall, runoff and substrate moisture content in the instrumented green roof test beds. Once validated, the thermal remote sensing techniques will provide an operational method for characterising a wide range of SuDS vegetation types and settings across the city. The remote sensing will utilise both a UAV (drone) and a sensing vehicle equipped with thermal and hyperspectral cameras to provide high-resolution data on vegetation coverage and on the plant canopy's thermal and water status. Finally, the acquired data will be utilised to produce practical SuDS design guidance.
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