Using the Urban Tethys-Chloris (UT&C) model to estimate the surface energy balance of different garden materials and configurations

Domestic gardens comprise up to 30% of urban area in the UK, providing many ecosystem services (ES), such as flood risk mitigation and temperature regulation, through vegetation present. Current estimates of ES provisioning using urban land surface models often focus on green space in an entire town/city, rather than specific greenspace types (Zawadzka et al., 2021), and often omit domestic gardens entirely, which may lead to unreliable recommendations. We chose the Urban Tethys-Chloris model (UT&C; Meili et al., 2020) to estimate ES delivery by domestic gardens because it considers both the energy and water balance, at the local scale, and allows for configuration and simulation of both vegetated and man-made surfaces. UT&C is a fully coupled energy and water balance model, that calculates 2m air temperature and skin temperatures of urban areas, accounting for biophysical and ecophysiological characteristics of ground vegetation and urban trees. Input meteorological data over the course of 2024 was obtained from the University of Reading Atmospheric Observatory (Reading, UK). Model garden plant species were specified as Lolium perenne (Perennial ryegrass; for ground vegetation) and Pyrus calleryana (Callery pear; for urban trees), and urban geometry values (such as house height and width) were specified as UK averages. We have found that UT&C realistically estimates seasonal and diurnal urban surface energy fluxes within a typical UK garden. Specifically, in summer, a garden made up of 100% vegetation (short lawn and 2 trees) had a peak surface temperature 13°C cooler, and a 2m air temperature 1°C cooler, than a garden made entirely of concrete. This is largely because vegetated ground loses heat through latent heat flux throughout the growing period, while impermeable surfaces can only do so after heavy rainfall (when water ponds on the surface). Gardens with 100% granite, concrete and slate surfaces had a surface temperature up to 6°C lower and a 2m air temperature 0.5°C lower than asphalt and wood decking as a result of their high thermal conductivity and heat capacity, suggesting these materials would be marginally better at maintaining a lower air temperature within an entirely impermeable garden, particularly in urban summers. Air and surface temperatures over semi-permeable materials, such as artificial turf and wood chips, were often higher than those found for impermeable surfaces, suggesting that they may not be an appropriate method of reducing air temperatures in gardens. Further work will focus on modelling the role of vegetation and garden surface choices on the surface water balance, and on translating the mechanistic model outputs into human comfort and flood mitigation indices. Additional models, such as SUEWS (Järvi et al., 2011), will also be used to estimate ES delivery, and to allow for model intercomparison. Following these simulations, we hope to provide recommendations to UK gardeners about the best hard landscaping materials, plant species, and garden configuration (e.g. proportion of trees versus lawn and bedding plants) to help reduce air temperatures and flooding within their neighbourhoods.