Evapotranspiration and green roofs as instruments for climate adaptation in urban areas

Initial situation
Climate change is progressing at an unprecedented rate worldwide and is affecting numerous social, ecological and economic systems. The effects are also clearly visible in Austria. Current climate data shows that the average annual temperature is rising more than twice as fast as the global average. Urban areas, agricultural land and alpine and forested areas are particularly affected.

Against this backdrop, adapting to climatic changes is becoming increasingly important. Green roofs are an effective instrument, especially in urban areas, for reducing thermal stress, regulating the water balance and improving the quality of life in cities.

Evapotranspiration as an indicator of climate resilience
Evapotranspiration describes the combined evaporation of water from the substrate and surfaces as well as the transpiration of plants. This process is closely linked to the energy and water balance of vegetation areas and acts as a natural cooling mechanism.

In the context of modern climate adaptation strategies, ET is increasingly being used as a key indicator for assessing climate resilience. It allows conclusions to be drawn about:

• the vitality of vegetation,
• the water stress of green spaces,
• the cooling capacity of greening systems,
• the effectiveness of green infrastructure.

Fig 1: Evaporation estimate based on satellite data over Vienna in September 2025 (c) Sistema

From satellite to map: How ET becomes visible
As part of the FFG project GET-ET, an innovative system for high-resolution estimation of evapotranspiration was developed. It is based on measurements from the ECOSTRESS satellite sensor and multispectral data from Sentinel-2.

ECOSTRESS provides daily ET maps with a spatial resolution of around 70 meters and serves as a reference for the modeling. In addition, terrain data and vegetation parameters from Copernicus data are used. The reference data is transferred to higher resolutions using a data mining/sharpening process.

Between 2019 and 2025, a comprehensive data set was created for Austria that takes atmospheric influences and temporal correlations into account. The data was integrated into a neural network based on a U-Net architecture with ResNet blocks. The use of perceptual loss enabled particularly precise ET estimates to be achieved. The model achieved a Structural Similarity Index (SSIM) of 0.91, which demonstrates a high level of agreement with reference data.

On this basis, an area-wide ET map can be generated for each new Sentinel-2 image, which is aggregated monthly and integrated into the GTIF platform.

Ensure quality, show impact
High-resolution ET maps are a valuable tool for planning and evaluating urban green infrastructure. Particularly in the area of green roofs, they enable an objective assessment of cooling performance and functionality.

ET analyses can be used to:

• urban heat islands can be identified,
• deficits in the green infrastructure are recognized,
• priority locations for green roofs and façades,
• the effectiveness of existing greening systems can be monitored.

Green roofs with high evapotranspiration contribute significantly to lowering surface temperatures and cooling the surrounding air. At the same time, they release stored precipitation water into the atmosphere with a delay and relieve the urban drainage system.

Another major advantage of ET-based analyses is the continuous monitoring of green roofs. Changes in evapotranspiration can be detected at an early stage:

• Drought stress,
• Vegetation damage,
• insufficient substrate moisture,
• indicate maintenance deficits.

This enables data-based quality assurance that supports the long-term preservation of the ecological and climatic functions of green roofs. As part of the GET-ET project, these approaches will be demonstrated and validated using practical use cases in Vienna, among other things.

Fig 2: Measurements – Example roof greening at the General Hospital (AKH) Vienna, 9th district (c) BOKU IBLB

The validation was carried out at two measurement sites in Vienna’s urban area (compact medium-height and large-scale low buildings), where a specially developed lysimeter setup (automatic pot lysimeter) was installed on green roofs. The lysimeters were designed in pairs and represented a shallow and a deep soil substrate (10 cm with an extensive plant mixture and 25 cm with an intensive plant mixture). Each set-up was equipped with load cells for mass balance, soil moisture sensors (top substrate layer; in the intensive set-up additionally in a deeper layer) and a meteorological station. The measurement campaign ran from May 2022 to December 2023 under field conditions and recorded a wide range of weather, water availability and phenological conditions.

The validation combines in-situ and satellite data at the pixel level of the roof: on cloud-free days, the daily lysimeter evaporation rate (ETa) is compared directly with the satellite-based ETa at 10 m resolution (ECOSTRESS-Sentinel-2) over the respective roof area. In addition, a model-based validation is carried out using the FAO-56-Penman-Monteith approach (Allen et al., 1998), which was calibrated using local meteorological measurements and plant-specific crop coefficients (Kc) derived from the lysimeter data.

Climate-resilient building practice – from data to decisions in planning & construction
The integration of evapotranspirational analyses into building and urban planning supports a holistic view of buildings as part of urban climate systems. This opens up new possibilities for building practice:

• Evaluation of greening concepts,
• Optimization of system superstructures,
• Adaptation of irrigation strategies,
• Support for normative and planning decisions.

ET data can thus serve as an objective basis for decisions on climate-adapted construction methods.

Conclusion: More impact through better data
Evapotranspiration is a key mechanism of action of green roofs and a reliable indicator of their climatic performance. The high-resolution ET products developed in the GET-ET project enable a comprehensive, regular and precise assessment of green infrastructure in Austria for the first time.

By combining earth observation data, modern modeling methods and practical applications, these systems make an important contribution to climate-resilient urban and building development. Green roofs can thus be planned, monitored and optimized in a targeted manner and form an essential component of sustainable building concepts of the future.

Literature

• Allen, R. G., Pereira, L. S., Raes, D., & Smith, M. (1998). Crop evapotranspiration – Guidelines for computing crop water requirements. FAO Irrigation and Drainage Paper 56.
• Berndtsson, J. C. (2010). Green roof performance towards management of runoff water quantity and quality: A review. Ecological Engineering, 36(4), 351-360.
• Fisher, J. B., et al. (2019). ECOSTRESS level 3 Evapotranspiration, Version 2. NASA EOSDIS.
• IEA (2025). Austria climate resilience policy indicator. International Energy Agency.
• Santamouris, M. (2015). Regulating the damaged thermostat of the cities – Status, impacts and mitigation challenges. Energy and Buildings, 91, 43-56.
• Wang, Z., Bovik, A. C., Sheikh, H. R., & Simoncelli, E. P. (2004). Image quality assessment: From error visibility to structural similarity. IEEE Transactions on Image Processing, 13(4), 600-612.