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• Increasing tree cover makes irrigation essential during a prolonged drought. • Irrigation helps alleviate water competition between different vegetation types. • Soil type strongly affects the water status and competitiveness of vegetation. Increasing urban green spaces and canopy cover requires careful planning of irrigation strategies, especially in arid and semiarid areas. This study investigates how vegetation cover and irrigation affect the water balance and vegetation productivity of a small urban reserve in the Melbourne metropolitan area, Australia. Using a mechanistic ecohydrological model, a series of numerical experiments were carried out for the period 1999–2018, which included a prolonged drought. Results indicated that irrigation played an essential role in helping both trees and grass productivity by increasing soil moisture and vegetation water access during the drought. With 10% tree cover, grass benefitted more than trees by increasing irrigation, and trees coped well with drought even without additional water. However, trees strongly relied on irrigation to maintain productivity when tree cover increased, highlighting the need for a sustainable balance between increasing urban greening and water conservation. Differences in soil properties and rooting strategies were also found to strongly modify the need for irrigation and the competition for water. These results provide quantitative insights on how increasing tree cover and vegetation diversity may impact irrigation requirements, highlighting the key role of mechanistic numerical models to support urban planners in the evaluation and design of urban green spaces. [ABSTRACT FROM AUTHOR]