Urban trees and rainfall: an investigation into the benefits for stormwater management
Creators: name: Dias Baptista, Mariana
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 330404
subject: Land use and environmental planning

description: Expanding urban areas have replaced the natural landscape. With reducing areas of natural space, evapotranspiration losses and infiltration rates have decreased, disturbing the natural hydrological cycle. As a result, the frequency of floods has intensified in those areas. As part of an integrated solution, city planners have promoted an increase in vegetated areas to <br>enhance evapotranspiration and infiltration rates and, consequently, reduce the runoff effect. In particular, trees play an important role, intercepting water on their leaves and branches during rainfall events and reducing the volume of water that generates runoff. The intercepted volume is directly connected to plant area density, which varies from one species to another, <br>but also from one individual tree to another. Variations in plant area may occur for different reasons during tree life duration, such as severe drought, heat waves, diseases and pruning. However, the effect of this variation on runoff reduction has not been tested. The present study evaluates the interception process for different trees planted in the City of Melbourne, analysing the impact of species-specific traits and variations in plant area on water storage and spatial-temporal redistribution. Measurements are taken by two different methods: first, as an indoor experiment, where rainfall is simulated and environmental conditions are controlled; and second, as an outdoor experiment, where throughfall is measured in an urban park. In the indoor simulated rainfall experiment, measurements are taken of Cmax, the maximum volume of water that a tree can carry on its surfaces while it is raining, and Cmin, the maximum volume of water that the tree carries when rainfall and dripping have ceased. Three different tree species commonly planted in Melbourne streets and parks (Ulmus procera, Platanus x acerifolia and Corymbia maculata) were studied. Leaf area was manually varied through staged leaf removal, creating four different leaf-density treatments for each tree: full canopy (100% of leaves), half (approximately 50%), quarter (approximately 25%) and woody (no leaves). Additionally, throughfall redistribution is analysed for the same trees with their full canopy. Terrestrial laser scanning (TLS) data is used alongside directly quantified leaf and branch area data to assess the capacity of TLS to predict canopy area metrics and associated canopy interception parameters such as Cmax and Cmin. TLS data is also correlated against throughfall distribution on a sub-canopy scale to investigate the predictive capacity. In the outdoor natural rainfall experiment, three tree species commonly planted in Melbourne parks (Ulmus procera, Ficus macrophylla and Eucalyptus microcorys) are analysed for their potential to intercept rainfall and delay throughfall in urban areas. Similarly, tree metrics are assessed by hemispherical photography and TLS. The collected throughfall and tree metrics are used to predict storage capacity using existing models for canopy interception prediction. Simulation results show that canopy storage capacity is well correlated to plant surface area (m2), plant area index (m2/m2) and plant area density (m2/m3) under controlled conditions. All analyses indicate that U. procera is the most efficient species for storing rainfall water within a canopy of equal volume or area index. The outdoor experiment confirms the importance of leaf density on storage capacity, but also the influence of different leaf and branches characteristics, as F. macrophylla presents the highest interception rates, but not the highest values for leaf area density. Analysing the pattern of throughfall under canopy reveals that throughfall and plant density are weakly related, but overall areas of reduced throughfall are associated with the areas of denser canopy above. Additionally, patterns of distribution vary from one species to another, which may be attributable to the size of leaves and distribution of branches and leaves in the canopy volume. This study contributes to the discipline and practice of urban forestry by distinguishing how variations in leaf density are important to consider when selecting tree species to be planted in urban street and greenspace landscapes, as well as the importance of using adequate management techniques to guarantee good health conditions for urban trees. These findings may provide guidance in relation to the use of trees in stormwater management policies, planning to achieve a higher potential for interception and runoff reduction for mitigating flood occurrences.