Evapotranspiration of urban street trees is essential in mitigating urban heat islands due to its cooling effect. However, current shifts in rainfall and temperature regimes towards drier and hotter periods in Central Europe have caused substantial water stress for street trees. Quantifying and subsequently managing these changing dynamics as well as estimating evapotranspiration and water availability is necessary but at the same time extremely challenging in urban environments. Both dynamics are influenced by soil sealing and complex shading patterns of the surrounding street canyon, which vary on a small spatial scale as a function of the canyon layout and orientation. In the present study, the diurnal patterns of six typical urban shading types for street trees were derived by considering a large set of street orientations, widths and tree positions within the street canyon. A shading model was integrated into a hydrological urban tree model to assess the impact of those shading types on diurnal patterns of radiation and evapotranspiration rates calculated using the Penman–Monteith approach and the resulting soil moisture conditions for several vegetation seasons and water‐supply scenarios. The modelling results showed that the six shading patterns significantly influenced the simulated hourly, daily and seasonal potential and actual evapotranspiration rates and water availability. Shaded trees have a substantially reduced, simulated water stress period, regardless of initial water supply, and are able to provide a longer‐lasting cooling function during dry periods due to higher evapotranspiration rates later in the summer season.
<p>Green roofs are promoted to deliver climate regulation and urban heat island mitigation and many more. Ecosystem services of green roofs are discussed generally positively, but their footprint from production and demolition processes have not been fully addressed. In this study, a life cycle analysis (LCA) was conducted to assess the possibility of creating of a carbon-neutral green roof and to evaluate and compare the global warming potential (GWP) of two green roofs: 1) a conventional green roof (GR-c) with expanded clay, pumice, and compost in the substrate and a polypropylene drainage, and 2) an eco-friendly green roof (GR-a) with recycled bricks and compost in the substrate and a cork drainage. The LCA refers to a functional unit (FU) of a 218 m<sup>2</sup> green roof (substrate depth of 9 cm; lifespan of 40 years). The results showed that the use of a brick substrate can reduce the GWP to 3139 kg of CO<sub>2</sub> eq/FU (- 50%) and the use of cork drainage to 441 kg of CO<sub>2</sub> eq/FU (- 69%). Apart from production, demolition is a key process&#160; to be improved in future, accounting for 32% (GR-c) and 55% (GR-a) of the GWP. Once produced, a green roof can take up 783 gCO<sub>2</sub>/(m<sup>2</sup>&#8901;a) because of plant uptake. To become CO<sub>2</sub>-neutral, a GR-c and GR-a would have to last 88 and 53 years, respectively. Furthermore, the GWP was influenced by green roof maintenance and plant CO<sub>2</sub> uptake. We conclude that recycled bricks and cork are promising green roof materials.</p>
<p>Current shifts in rainfall and temperature regimes towards dryer and hotter periods in central Europe have caused substantial water stress for urban trees. &#160;To be able to adapt water supply to urban trees under a changing climate, &#160;a quantification of evapotranspiration and water availability becomes necessary and is at the same time, very challenging in the heavily modified urban environments. Both processes are influenced by soil sealing and complex shading patterns of the surrounding street canyon. &#160;</p> <p>For five urban street trees in the city center of Berlin, evapotranspiration rates and water availability was monitored in a field campaign (sapflow measurement and soil moisture in different depth) during the vegetation period of 2022. The monitoring results were then used to test a hydrological urban tree model with an integrated shading model which specifically takes into account the shading and sealing variability of the surrounding built environment.</p> <p>Both measured and modelled data a &#160;show that potential evapotranspiration rates were significantly larger for trees with full sun exposure compared to shaded trees. At sites with full sun exposure, the increased evapotranspiration also reduced soil moisture content faster; at the same time measured actual evapotranspiration was reduced by up to 2/3 during water stress periods.</p> <p>In conclusion, the comparison showed that our model is a promising option to obtain information on water availability and to improve water management for urban trees under different shading and sealing environments in heavily modified cities. The tool will be further developed to be used by local authorities and practitioners to identify water shortage periods and hot spots &#160;within the city to optimize irrigation efforts.</p> <p><strong>Key words: urban trees, evapotranspiration (ET), water availability, water stress, water management, &#160;urban environment, shading </strong></p>
<p>Evapotranspiration (ET) is a key parameter in the water exchange of atmosphere, plant and soil and was studied on many different scales. In urban environments, the estimation of evaporation is particularly difficult, as it is effected by complex patterns of shading, which varies on a very small scale as a function of street canyon layout and orientation. Moreover, shading varies not only in space but also in time due to the seasonal orientation and altitude of the sun. Therefore, for a correct ET assessment, the diurnal variations as well as the annual variations of shading must be taken into account. For this purpose, radiation is divided into direct and diffuse radiation; in case of complete shading only the diffuse radiation was used for ET estimation, reducing the direct radiation to zero. The diffuse radiation is further influenced by the amount of visible sky in a street canyon as a function of street widths, which can be derived using the sky view factor. <br>To reduce the uncertainty of ET estimation in the built environment, a process-based model was developed with an hourly resolution that takes into account the particularly heterogeneous spatial variability of urban surfaces. To assess the impact of shading on ET, six different shadow scenarios as well as two typical urban soil sealing scenarios for a wide and a narrow street canyon were analysed regarding differences and similarities of radiation and resulting actual and potential ET of a street tree as well as soil water dynamics. <br>The model scenarios showed that ET is highly influenced by shading. Furthermore, shadow scenarios affect actual ET (ETA) differently during the vegetation period: whereas in April the ETA is higher for fully exposed sites, this changes by June when less exposed sites periodically have higher ETA rates. This difference is directly connected to alteration of soil moisture dynamics, for a fully sun exposed site a soil moisture of 10 Vol% is already reached by June. For a shaded site the decrease to 10 Vol% takes two months longer.</p><p>In conclusion, the results highlight that it is essential to include the effects of shading in the quantification of vertical water fluxes in urban environments. Moreover, this new model approach will help to identify water shortage periods and critical locations for street trees.</p>
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