An urban ecohydrological model to quantify the effect of vegetation on urban climate and hydrology (UT&C v1.0)

Increasing urbanization is likely to intensify the urban heat island effect, decrease outdoor thermal comfort, and enhance runoff generation in cities. Urban green spaces are often proposed as a mitigation strategy to counteract these adverse effects, and many recent developments of urban climate models focus on the inclusion of green and blue infrastructure to inform urban planning. However, many models still lack the ability to account for different plant types and oversimplify the interactions between the built environment, vegetation, and hydrology. In this study, we present an urban ecohydrological model, Urban Tethys-Chloris (UT&C), that combines principles of ecosystem modelling with an urban canopy scheme accounting for the biophysical and ecophysiological characteristics of roof vegetation, ground vegetation, and urban trees. UT&C is a fully coupled energy and water balance model that calculates 2 m air temperature, 2 m humidity, and surface temperatures based on the infinite urban canyon approach. It further calculates the urban hydrological fluxes in the absence of snow, including transpiration as a function of plant photosynthesis. Hence, UT&C accounts for the effects of different plant types on the urban climate and hydrology, as well as the effects of the urban environment on plant well-being and performance. UT&C performs well when compared against energy flux measurements of eddy-covariance towers located in three cities in different climates (Singapore, Melbourne, and Phoenix). A sensitivity analysis, performed as a proof of concept for the city of Singapore, shows a mean decrease in 2 m air temperature of 1.1 ∘C for fully grass-covered ground, 0.2 ∘C for high values of leaf area index (LAI), and 0.3 ∘C for high values of Vc,max (an expression of photosynthetic capacity). These reductions in temperature were combined with a simultaneous increase in relative humidity by 6.5 %, 2.1 %, and 1.6 %, for fully grass-covered ground, high values of LAI, and high values of Vc,max, respectively. Furthermore, the increase of pervious vegetated ground is able to significantly reduce surface runoff.

Tree effects on urban microclimate: Diurnal, seasonal, and climatic temperature differences explained by separating radiation, evapotranspiration, and roughness effects

Paolo Burlando, Matthias Roth

Increasing urban tree cover is an often proposed mitigation strategy against urban heat as trees are expected to cool cities through evapotranspiration and shade provision. However, trees also modify wind flow and urban aerodynamic roughness, which can potentially limit heat dissipation. Existing studies show a varying cooling potential of urban trees in different climates and times of the day. These differences are so far not systematically explained as partitioning the individual tree effects is challenging and impossible through observations alone. Here, we conduct numerical experiments removing and adding radiation, evapotranspiration, and aerodynamic roughness effects caused by urban trees using a mechanistic urban ecohydrological model. Simulations are presented for four cities in different climates (Phoenix, Singapore, Melbourne, Zurich) considering the seasonal and diurnal cycles of air and surface temperatures. Results show that evapotranspiration of well-watered trees alone can decrease local 2 m air temperature at maximum by 3.1– 5.8 °C in the four climates during summer. Further cooling is prevented by stomatal closure at peak temperatures as high vapour pressure deficits limit transpiration. While shading reduces surface temperatures, the interaction of a non-transpiring tree with radiation can increase 2 m air temperature by up to 1.6 – 2.1 °C in certain hours of the day at local scale, thus partially counteracting the evapotranspirative cooling effect. Furthermore, in the analysed scenarios, which do not account for tree wind blockage effects, trees lead to a decrease in urban roughness, which inhibits turbulent energy exchange and increases air temperature during daytime. At night, single tree effects are variable likely due to differences in atmospheric stability within the urban canyon. These results explain reported diurnal, seasonal and climatic differences in the cooling effects of urban trees, and can guide future field campaigns, planning strategies, and species selection aimed at improving local microclimate using urban greenery.

2021

Towards Solar Decathlon: Concepts for a Sustainable Urban Water Management in a new eco-district

Gianluca Antonio Paglia

Water is rarely considered to be a crucial aspect in sustainable housing. In fact, water sustainability in eco-districts and in ecological architecture is frequently limited to recycling and reuse solutions. Why less water? What could explain the choice of an installation, of a specific strategy or of a criterion trying to decrease water needs? Terms as ecological footprint or sustainability are becoming more and more used, however, how can sustainability be measured? What does sustainable water mean? What are the parameters that quantify the ecological footprint of a water management system? Within this project, some possible concepts of optimization and valorization of sustainable urban water system have been defined and explored. Numerous studies about water management, water distribution, sanitary technologies, recycling, etc. have already been done. Nevertheless, it is less common to identify the reasons that are able to justify a reduced consumption of water. Eco-district labels propose several solutions for sustainable water, however they are limited by scale or simply focused on one stage of the water cycle. The aim of this project is then to consider water from a larger point of view: through a Life Cycle Analysis approach, needs and properties of different urban water management systems are compared and evaluated. The scale considered for such reflections is effectively a central parameter; in this case, hypothesis will be applied to collective habitations inserted in a district and part of a specific urban context: the city of Fribourg. The urban environment has particular features in relation to surrounding rural areas: energy exchanges, hydrology characteristics and limited ecosystems. The relationship between the use of natural resources and urban development is becoming more and more important; the purpose of this thesis is then to define a methodology able to suggest a sustainable use of water through environmental, political and economic issues. A real urban case study with its current water management system defines what has to be analyzed and allows evaluating the limits and the possible enhancements of such system. Thus, in order to have an idea about the potential of sustainable water management solutions, an ambitious analysis of energy related to water uses and disposals is created and is used to show the reasons for change in a real model as the city of Fribourg. A comparison between two extreme scenarios of Fribourg’s selected zones is finally developed, discussed and implied in the conclusions through specific actions and facts. The results show that a high potential of improvements exist. Using them to elaborate new strategies in collaboration with the several stakeholders contacted may contribute to achieve many of today’s objectives for a sustainable urban water management. The best scenario, chosen with a new concept of eco-label, summarizes and concludes the analysis.

2015

Optimization based approach to assist the transition of large energy users to low temperature networks

Francesca Belfiore, François Maréchal, Alberto Mian, Jessen Page

Distric Energy System (DES) represents a promising opportunity to improve energy efficiency and boost the decarbonization of the European energy sector. If most of the studies focus on small consumers typically located within urban areas, large energy users like hospitals, university campuses or airports, constitute complex and unique systems with great potential for energy and cost savings. The aim of this work is to propose a robust method to optimize the preliminary design and operation of local DES characterized by complex non-residential buildings. Decision variables include the optimal set of technologies to be installed (type, size, location), their operating schedule, together with temperature and configuration of the thermal network. Focus is given to heat pumping technologies and in particular to the comparison between different degrees of centralization and respectively, decentralization. The mathematical model is based on a Mixed- Integer Linear Programming (MILP) formulation including a set of heat cascade constraints to ensure the feasibility of the heat exchanges. A case study is generated to represent a building-stock constituted by large non-residential buildings, connected to a central plant through an existing network infrastructure. The users differ for power and temperature demand, both estimated as linear function of the ambient temperature. Results show that the optimum degree of decentralization is highly influenced by the distribution of the heat demand among the users at the different temperatures. Therefore, the use of local heat pumps to upgrade the heat produced by a central unit does not always translate in operating cost savings. For the case study presented in this work it is shown that if the lowest temperature user covers 66% or more of the total demand a decentralized configuration is the most cost effective solution.

Curran Associates, Inc.2020