School of Agriculture, Food and Ecosystem Sciences - Theses
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ItemStormwater runoff retention and tree growth in passively irrigated street tree systems( 2023-11)Urbanisation creates extensive impervious surface cover which alters natural hydrological cycles. Impervious surfaces generate large volumes of stormwater runoff, which degrades receiving waterway ecosystems when conventionally drained. To protect urban streams, runoff volumes need to be significantly reduced to restore pre-development flow regimes. Street trees have considerable potential to increase the demand for stormwater, by transpiring large volumes of water if irrigated with runoff via ‘passive irrigation’. Passive irrigation systems can also benefit trees by reducing drought stress and increasing growth and therefore providing shading and cooling benefits. However, lack of knowledge on system design and tree species selection is preventing widespread uptake of passive irrigation systems for street trees. This thesis aimed to identify the drivers of runoff retention, tree growth, water use and drought response, to inform the design of and tree species selection for passive irrigation systems. Runoff retention and tree growth were quantified for alternative system designs with different inlet types and storage volumes in the field. Using this data, runoff retention was also modelled for a range of soil exfiltration rates, climates and storage sizes. Tree water use, drought response and growth were assessed in a glasshouse study to inform species selection in relation to system storage. In the field, systems with inlets designed to exclude sediment captured less runoff. However, the exfiltration rate of the surrounding heavy clay soil ultimately limited runoff retention. Therefore, inlet selection will likely be driven by maintenance requirements and cost, rather than runoff retention. However, where storage volume was large relative to the contributing catchment area, or where exfiltration rates were faster, runoff retention was substantially greater. Passively irrigated trees in the field grew faster than control trees; however, this was likely due to trees having both adequate drainage, to avoid waterlogging, as well as an internal water storage. In the glasshouse study, trees with an internal water storage and low stomatal sensitivity showed lower drought induced leaf loss compared to those without, whereas trees with high stomatal sensitivity and an internal water storage showed lower drought stress and recovered rapidly after drought. This thesis demonstrated that passive irrigation systems have significant potential to reduce runoff volume entering waterways. However, large storage volumes will be required to facilitate exfiltration in heavy-textured soils. Passive irrigation systems can improve tree growth when waterlogging is avoided and internal water storage provided, which will likely increase evapotranspiration, runoff retention and canopy growth in the future. As per other stormwater control measures, maintenance requirements must be considered during design and greater oversight during construction is required to ensure systems perform as intended. Designs need to flexible and adapt to local soil and climate conditions. Matching tree species, or more accurately, tree water use strategies, to system design is required to ensure systems maintain function in future climates.