In general, cities have developed and expanded in places where natural and environmental conditions were the most favourable, and they can only continue to prosper by conserving the natural resources that are the drivers of their wealth and quality of life. Four out of five European citizens live in urban areas and the quality of life is greatly influenced by the urban environment. Here, gardens, parks and green open spaces improve air quality and reduce the impact of stormwater events by reducing the value of the runoff coefficient; they offer lower temperatures in hot urban islands and represent habitats for biodiversity. Gardens in many cities, such as our case study of Coimbra, were initially built for decorative, architectural, and leisure purposes, or with specific academic goals in mind, but their size or urban relevance often made them important factors in city planning and resources' sustainability, as well as for the wellbeing of their citizens. Sustainable water management in urban areas involves promoting rational water use, and also the identification of strategic reserves to deal with droughts when they occur. By improving the management of the urban water cycle as a whole a more efficient use of resources can be achieved, providing not only economic benefits but also improving social and environmental outcomes. Because water is a limited and scarce resource it must be used in gardens in an efficient and sustainable way. For this reason grasses, trees, shrubs and flower beds must be provided only with enough water to satisfy their needs. Using the Landscape Coefficient Formula, the amount of water needed for irrigation can be expressed as a percentage of reference evapotranspiration (ET0). The value of evapotranspiration should be adjusted to best meet the water demands of a given assortment of plant species. In this paper we present a study performed at the Botanical Garden of the University of Coimbra (BGUC). Monthly and annual levels of precipitation were evaluated along with the air temperature and evapotranspiration to determine the climatological water balance through water shortages and surpluses. The results were compared with the levels of water consumption (from the water supply and spring water collector) and the efficiency of irrigation was assessed, adjusted for plant type, enabling the identification of procedures and opportunities to maximize the efficient use and sustainable management of water.

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