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In the center of the city the Urban Heat Island is the worst because of its nature as the focal point of development and traffic intensity.

The center of the city also supplies the most population & employment density.

Therefore, we need to understand that grass and thick shade trees are both crucial and efficient for these areas in and around downtown.
~Crucial because of the increasing need to mitigate the Urban Heat Island.
~Efficient because, due to the nearby density, the water use for this type of landscaping benefits the most people and leads to an efficient water use per capita ratio even when the landscaping requires more water than xeriscaping.

Density has its benefits. This is one of them. So please stop applying water usage guidelines the same to all built environments.