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

Abstract

Abstract. 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.

A single tree model to consistently simulate cooling, shading, and pollution uptake of urban trees

“…A possible explanation is that foliage and thus transpiration demand might increase faster in reality than assumed in the model which would call for a more comprehensive determination of surface temperature. Also, some differences are apparent between the development of soil water potential and the simulated soil water content, indicating either a very inhomogeneous precipitation distribution within the city or a non-linear relation between runoff and percolation that might need to consider ponding or channeling effects from gutters (Meili et al 2020 ). The very scattered distribution of sensors, the scarce information about the soil properties, and the assumption of homogeneous soil sealing, however, demand for further evaluation studies in order to determine the importance of different foliage, surface, and soil properties to improve the respective parameterization.…”

A Multi-Layer Model for Transpiration of Urban Trees Considering Vertical Structure

“…where a = 0.01 ms −1/2 is an empirical coefficient [46], d lea f (m) is the characteristic leaf dimension, often referred to as the leaf width, and u(H k ) ms −1 is the wind speed at each layer height H k . The wind speed profile is assumed to be logarithmic above the urban canopy and exponential within the urban canyon using Equations (5) and (6) [35,48,49] u…”

“…H c (m) is the canopy height, λ p (−) is the plan area index of the urban roughness elements, A f ,b (m) is the actual frontal area of buildings, A f ,v (m) is the actual frontal area of vegetation, A tot (m) is the total urban plan area, and P v (−) is the ratio between vegetation drag C Dv and building drag C Db . These parameters were calculated from [35,[52][53][54] using the height of trees and buildings. For volumetric/aerodynamic porosity, the light extinction parameter is calculated as given by [17], assuming a spherical leaf angle distribution.…”

Quantifying the Independent Influences of Land Cover and Humidity on Microscale Urban Air Temperature Variation in Hot Summer: Methods of Path Analysis and Genetic SVR

“…Taking into account the replacements of natural land cover in the city, the increase in urban vegetation is often a recommended strategy for cooling urban areas and mitigating high air temperatures [27][28][29]. However, increasing vegetation is usually accompanied with an increase in humidity [30,31]. It is difficult to distinguish between the contributions of vegetation and humidity increases to temperature decreases.…”