What Do Street Trees Do When It Rains?

Have you ever watched a road in heavy rain?

Water lands on the asphalt, runs along the edge of the roadway, and drops into a storm drain. A forest behaves differently. When rain falls there, the leaves catch it first, and the water that does reach the ground soaks into the soil.

Same rain. Why so different?

Cities are full of surfaces water cannot pass through: asphalt, concrete, rooftops. These are called impervious surfaces.

When rain falls on soil, some of it sinks in. On asphalt and concrete it cannot. The water that fails to soak in flows across the surface as runoff, and runoff from roads and buildings goes into storm drains and on into the sewer network.

On an ordinary day this looks like no problem at all. But when a lot of rain falls in a short time, water from across a wide area arrives at the drainage system all at once.

Urban flooding depends on many things — how much rain falls, the shape of the land, the capacity of the pipes. And one of them is how much space the city has where water can soak in.

Ask what street trees do and most people think of shade, or cleaner air. But trees are working on rainy days as well.

The first thing is interception. Some of the rain does not fall straight to the ground; it stays on leaves and branches, and part of that evaporates back into the air.

The second is infiltration. If there is soil around the tree, some of the water that reaches the ground soaks into it.

The third is transpiration. A tree draws water up through its roots and releases part of it back into the air through its leaves.

So a street tree catches the rain first → lets it soak into the ground → sends some of what it absorbs back into the air, and in doing so changes how water moves through the city.

So how much are Seoul's street trees actually holding back?

In 2024, researchers at Hanyang University analyzed 29,451 street trees in Seoul's Gangnam and Gangbuk districts, running the species, size, leaf area and Seoul's weather records through a model called i-Tree Eco.

The result: the street trees of Gangnam district were estimated to reduce about 3,977 m³ of runoff a year. That is roughly 3.97 million liters.

Which is still hard to picture. A pool 25 m long, 10 m wide and 1.5 m deep holds about 375 m³. So 3,977 m³ is enough to fill about ten 25-meter pools.

This figure is a model estimate, not water collected and measured in the field. Even so, it shows that the trees we walk past every day add up to a meaningful amount of water.

Street trees catch rainwater using their leaves and the soil around them. But a city has far more road and sidewalk than it has tree pits.

What if, beyond the trees, a city made more places where rainwater could sit and soak in? Parts of a road or sidewalk could be rebuilt to let water through, or small patches of green could hold rainwater for a while.

That idea has a name: Low Impact Development (LID). The term sounds technical, but the principle is simple. Instead of rushing all the rain into the sewer, store it and let it soak in close to where it fell. Rain gardens, permeable paving and vegetated swales are all ways of doing it.

One of them is the rain garden. It is what the name says: a garden that takes the rain. It looks much like an ordinary planting bed, but the ground is set slightly lower than its surroundings and it is built so that water running off the road or sidewalk can flow in.

When it rains, water enters the garden. It sits among the plants and soil for a while, then slowly soaks into the ground.

Road runoff → rain garden → temporary storage → infiltration → whatever is left drains away

On a normal day it is a small patch of green with trees and plants growing in it. On a rainy day it is a place that takes water in. The planting space around a street tree, or a strip of green along a road, can be used the same way.

What happens when this is applied to a real Korean neighborhood?

A 2020 study looked at a repeatedly flooded area of Juan-dong in Incheon, asking how runoff would change if LID facilities were installed. It built several scenarios using rain gardens, permeable paving and infiltration structures.

Depending on the scenario, annual runoff fell by roughly 29–45%. Among the facilities compared, rain gardens captured the most rainfall.

This was modeling, not a measurement taken after rebuilding an entire district — it used Juan-dong's land use and rainfall conditions. But it shows how much a city's water can change when its small spaces are turned into places where rain can rest and soak in.

Back to where we started. When rain falls on a city it runs along the road and into a storm drain, and conventional drainage takes that water and moves it away fast, through pipes.

Rain → road → storm drain → sewer

Green infrastructure — street trees, rain gardens — handles it differently. Before sending the water away, it holds some of it close to where it fell.

Rain → trees, soil, rain gardens → storage and infiltration → whatever is left drains away

One rain garden does not handle a city's stormwater. But the soil around street trees, small planting beds, parks and sidewalks can each take a share of it. That possibility is what the Incheon study points to.

Seen this way, a street tree looks a little different. It makes shade and cleans the air — and it also catches rain and lets it into the soil, as part of a city's water cycle. Design the space around the tree as well, and that role gets larger.

The infrastructure that manages a city's rain does not have to be a giant pipe underground. A single tree and a small patch of green, the ones we walk past every day, can be green infrastructure too.