Water cycle in the city

This section connects to exercise 5 Forest models

In a large city, a single tree can capture and redistribute an average of 6500 liters of rainwater each year. Without city trees, all of the water would fall to the ground, and much of it would become runoff. During a heavy rainstorm, the sewers could overflow from pollutant carrying water running too fast on the paved streets into nearby waterbodies and overflowing the city. But, with the help of trees, the water would be captured, stored and re-used as part of the natural water cycle, soaked up by trees and returned to the atmosphere.

In a large city with estimated 200 000 street trees, 400 000 park trees and hundreds of thousands of privately owned trees, it is an impressive amount of water that gets captured by the urban tree canopy. The more trees we add to our landscape, the less pollution will flow to our waters.

On the other hand, there often is a lack of water for long periods, because the city with paved surfaces can’t soak up the water. Trees and green surfaces are part of the solution for this problem. Plants and trees hold water for a long period of time. More green means less flooding, less excess heat and less droughts. In addition, greener cities are nicer to live in as they are more pleasing to the eye. A city needs paved surfaces to function, but many opportunities exist to make a city greener. For example, it is a good option for streets and parking lots to build them out of water permeable stones.

Citizens themselves can also play a role in storing water in the city. They can turn their front and back yards into gardens by removing pavement. Individual house owners without a garden can also contribute to a green environment. They can opt for ‘green roofs’ or to having a ‘pavement garden patch’. Some cities even encourage such citizen initiatives by granting subsidies for them. In the exercise ‘Forest models’, students will pour water through different substances. The results will be that the green substance holds up the most water. The other substances will either allow most of the water to flow through lose stones and sand) or retain the water on top of the surface without it infiltrating the material (clay).

Picture from: https://huisjetuintjeboompje.be/exterieur 2/tuin/zelf geveltuin aanleggen/
Photo by Egor Gordeev at Unsplash