Urban Ecohydrological Modelling to Quantify Vegetation Effects on Urban Climate and Hydrology

At present, more than half of the world’s population is living in cities and many regions of the world will likely experience further rapid growth in urban infrastructure. Such an increase in urbanization combined with the effects of climate change alters energy and hydrological fluxes, likely exacerbating urban heat islands (UHI), decreasing outdoor thermal comfort (OTC) in the hot regions of the world, increasing surface runoff, and modifying the availability of water resources. These changes in the urban hydro-climate exposes many people to heat and water related risks, decreases their well-being, and potentially causes feedback loops on energy consumption. Nature based solutions, such as the increase in urban green cover, are an often proposed mitigation strategy to address these problems of urbanization. However, the quantification of vegetation effects on urban climate and hydrology is lacking, and many urban climate models did not include or simplified vegetation and hydrological processes. The research presented in this thesis addresses this knowledge gap through (1) the development of a mechanistic urban ecohydrological model that accounts for the detailed biophysical and ecophysiological characteristics of urban vegetation and their effects on urban climate and hydrology. Subsequently, the newly developed model is applied to (2) quantify and analyse important mechanisms that govern the effects of tree cover on urban temperatures, and to (3) answer the question to which extent a change in urban vegetation cover and plant traits can provide an improvement of OTC in a tropical city. The newly developed urban ecohydrological model, Urban Tethys-Chloris (UT&C), combines principles of urban canopy with ecohydrological modelling (Chapter 2). UT&C is able to account for the biophysical and ecophysiological characteristics of ground vegetation, urban trees, and green roofs, and their interaction with the urban energy and water budget, including soil moisture at different depths and underneath varying surface types. UT&C calculates transpiration as a function of plant photosynthesis, which is controlled by vegetation physiology and environmental conditions. The model further accounts for interception on plant canopies, ponding on soil and impervious surfaces, urban irrigation, heat transfer into and out of buildings, and anthropogenic heat release. The performance of UT&C is assessed through a comparison of simulated and measured energy fluxes in neighbourhoods in Singapore, Phoenix, and Melbourne, three cities with a distinctively different climate. Subsequently, the daily and seasonal cycle of urban temperature alterations due to street trees are explained through the separate analysis of the tree-radiation interactions, tree evapotranspiration, and urban aerodynamic roughness alterations caused by trees (Chapter 3). The different tree effects are quantified through a numerical experiment conducted with UT&C. The analysis identifies that a non-transpiring tree canopy could potentially increase urban air temperatures due to a large release of sensible heat, even though it provides shade to the underlying surfaces. This is an important mechanism to consider as high vapour pressure deficits in cities during times of heat can lead to plant stomatal closure and therefore, limit transpiration as shown in the simulations. On the other hand, tree evapotranspiration leads to air temperature cooling. These results highlight the importance of ample soil moisture to sustain cooling benefits of urban vegetation, which might require irrigation at times of water scarcity. Last, a large increase in street tree cover can alter the urban aerodynamic roughness, which influences the convection efficiency of the turbulent transport of heat, and thus urban temperatures. UT&C is further applied to quantify the effects of urban vegetation fraction, type, and properties on OTC in a tropical city (Chapter 4), which is not straightforward to predict due to the increase in humidity caused by vegetation. A variance based sensitivity analysis of UT&C’s vegetation parameters shows that vegetation fraction has the highest influence on OTC. An increase in tree cover fraction can partially mitigate low OTC during daytime, while increasing vegetated ground fraction improves OTC at night. However, when analysing air temperature during midday, tree physiology can be as important as tree cover fraction. Furthermore, all vegetation parameters combined can alter OTC as much as the vegetation cover fraction in dense midrise areas. Overall, the average OTC improvement caused by urban vegetation in a tropical city is modest due to the increase in humidity, when considering all weather conditions. However, an increase in vegetation cover does not deter OTC in most cases. Lastly, an outlook on future model developments is provided, such as the inclusion of additional vegetation processes into UT&C and integrating it into larger scale models, which allows to simulate and address further areas of the urban eco-hydro-climatic environment