How do city trees get enough water?

How do large trees in cities get enough water to live on when most of the area around them is paved with concrete and asphalt?

Peter Peters

Sherborne, Dorset, UK

Virtually no paving construction is impervious to water.

Rain and water running along the street will percolate through cracks in concrete that are caused by ground movement. Water will also pass through joints where the paving meets other construction and soak through many coarse or bituminous surfacing materials.

Depending on the nature of the subsoil, water will diffuse laterally to reach tree roots, particularly tap roots of some species that are set deep. Some geological formations will also allow lateral diffusion over considerable distances, even many kilometres.

Driving rain falls on leaves and twigs and often stays on these convex surfaces because of the tendency of a jet of fluid to stay attached to a convex surface – known as the Coanda effect. It then runs down the bark of branches, limbs and trunks and can percolate through the small area of loose material directly over the root mass.

Many species of tree can also absorb rainwater through their leaves in order to survive.

If you observe the soils arising from public utility excavations of paving, you may be surprised by how moist they are.

Tony Goddard

Trelech, Carmarthenshire, UK

In the late 1980s, I worked for what was, at the time, utility company Eastbourne Water.

Back then, one of the water sample collectors told me that a lot of trees got by because of the number of leaks from the mains pipes. He even said that repairing the pipes led to trees dying. I hope this isn’t so much the case now, given the need to conserve water.

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