Why is green infrastructure becoming so important now? Our urbanized areas have greatly expanded to accommodate growing populations. But this growth has come at the cost of forests, wetlands, and undisturbed soils capable of managing stormwater naturally. As a result, water movement in the city is much different from in a field or forest. This is primarily due to the permeability of the surface the rain is falling on.

Diagram showing the speed of water movement in clay versus sand

How water moves in the forest

When it rains in a forest or on fields, the rainwater soaks into the soil. The soil then stores a portion of the water, while the remainder moves slowly to streams or into groundwater reserves.

Once water is in the soil, the speed of its flow slows down. After most storms, the water that soaks into the soil stays there and is taken up by plants. Even after heavier rains, most of the rain will stay in fine-textured and well-developed soils, or move very slowly through them. When heavier rains fall on coarsely textured or sandier soils, the water will move through the soil more quickly as shown in Figure 1.

In either case, water is able to enter the soil because of the soil’s permeability. This movement of water through the soil is called “subsurface flow.” Subsurface flow moves rainwater to streams, lakes, and other surface waters. It can also move water to groundwater where it is stored for centuries. The storage of water in soil is a critical type of water storage, providing water for streams, plants, and people long after the rain has stopped.

If the water travels instead over surfaces such as city streets or bare hillsides, it can carry sediment, nutrients, and other contaminants with it. When this water enters healthy soil, the soil acts as a filter, removing sediment and many of the nutrients and contaminants the water has picked up.

Many of these particulates are absorbed onto soil surfaces where they can either be taken up by plants and soil organisms or incorporated into the soil, making them unavailable to water or living things. That filtered water that results then slowly and steadily recharges groundwater and river systems, helping to keep streams cool, clean, and consistently full for people, fish, and other organisms.


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