Surface runoff facts for kids

Runoff flowing into a stormwater drain

Surface runoff is when water flows over the ground instead of soaking into the soil. It's also called overland flow. This happens when there's too much rain, melting snow, or other water, and the ground can't absorb it fast enough.

Sometimes, the soil is already full of water. Other times, rain falls quicker than the soil can drink it up. Places like roofs and paved roads don't let water soak in, which also causes runoff. Runoff is a big part of the water cycle. It's also the main way water causes soil erosion. The area of land where runoff collects and flows to a single point is called a drainage basin.

Runoff can carry pollution. This pollution comes from human activities or natural sources like rotting leaves. Things like petroleum, pesticides, and fertilizers can be picked up by runoff. This can lead to water pollution in rivers and lakes. In cities, runoff often causes urban flooding, damaging homes and streets.

How Runoff Happens

Surface runoff from a hillside after soil is saturated

Water flowing over the ground without going into the soil is called surface runoff. This is different from water that flows in streams.

Snow and glacier melt create runoff in cold places. This usually peaks in spring or summer. The sun and air temperature affect how fast snow melts. In places without snow, rain causes runoff. But not all rain creates runoff. Light rain can be absorbed by the soil. Some special plants can absorb a lot of rainwater, preventing runoff even after heavy rain.

When Rain Falls Too Fast

This happens when rain falls faster than the ground can soak it up. It's common in dry areas where the soil can't absorb much water. It also happens in cities where pavement and buildings stop water from soaking into the ground.

When Soil is Full

If the soil is already completely full of water, and it keeps raining, the new rain will immediately become runoff. How wet the soil was before the rain started affects how quickly it becomes full.

How Wet the Soil Is Before Rain

Soil holds some water after it rains. This leftover water affects how much more water the soil can absorb. If the soil is already quite wet, it will get full faster during the next rain. Once the soil is full, runoff begins.

Water Flowing Underground

Sometimes, water soaks into the soil on a hillside. Then, it flows sideways underground and comes out closer to a stream. This is called subsurface return flow.

As water flows over the land, some of it might evaporate. Some might collect in small puddles. And some might soak into the ground later. Any water left eventually flows into a river, lake, or ocean.

How Humans Affect Runoff

Rain washing pollutants into local streams

Urban surface water runoff

Building cities increases surface runoff. Paved roads and buildings don't let water soak into the ground. Instead, water flows directly into streams or storm drains. This can cause erosion and siltation (when dirt builds up). More runoff also means less water soaks into the ground to refill groundwater supplies. This can make droughts worse, especially for farmers.

When human-made pollutants mix with runoff, it causes water pollution. These pollutants can reach rivers, lakes, and oceans. This changes the water and harms the plants and animals living there.

Changes in our climate, due to greenhouse gases, are expected to change how and when rain falls. This will directly affect how much runoff occurs.

City Runoff

City runoff is a big problem. It comes from streets, roofs, and other hard surfaces. It often carries pollutants like oil, trash, and chemicals into our waterways.

Farm Runoff

Willow hedge used to limit runoff

Soil erosion by water on farmland

Runoff from farms can carry fertilizers and pesticides into rivers and lakes. This can harm aquatic life and water quality.

Problems Caused by Surface Runoff

Erosion and Soil Loss

Runoff can wear away the Earth's surface. This is called erosion. The eroded material, called sediment, can be carried far away. There are four main types of water erosion:

Splash erosion: Raindrops hitting the soil can dislodge tiny soil particles.
Sheet erosion: Water flows evenly over the land, carrying away a thin layer of soil.
Rill erosion: Runoff starts to form small, narrow channels, like tiny ditches.
Gully erosion: If rills get bigger and deeper, they become large gullies. These can move a lot of soil quickly.

Erosion reduces how much food crops can grow. It takes away the fertile topsoil. This makes the soil less healthy for plants. Tiny soil particles in runoff can make water cloudy, blocking sunlight for underwater plants and animals.

Some farming methods, like shifting cultivation, can increase erosion. Modern industrial farming can also cause a lot of erosion. Using methods like no-till farming can greatly reduce soil erosion.

Environmental Harm

Runoff can carry pollutants to rivers, lakes, and underground water. This harms the environment and can affect human health. Some common pollutants are petroleum, herbicides, and fertilizers. When drinking water comes from rivers or lakes, pollution from runoff can make it unsafe or bad-tasting. Polluted water can harm fish and other aquatic animals. It can affect their ability to mate, lay eggs, and survive.

Runoff from forests can carry nutrients into lakes, causing too much algae to grow. This is called eutrophication. Runoff can also carry natural acids from forests, changing water quality.

If underground water (aquifers) is used for drinking, runoff pollution can contaminate it. Runoff can also spread pollutants from one area of soil to another.

Farm Chemical Runoff

Farmland runoff

Farm chemicals like nitrates, phosphates, and pesticides can be washed away by runoff. This happens if too many chemicals are used or if it rains heavily soon after they are applied. This wastes chemicals and harms the environment.

Economic Costs

Surface runoff causes economic problems. It can damage crops and reduce their value. For example, in China, runoff has affected vegetable crops. Farmers are trying new ways to keep nutrients in the soil. Pine straws and elephant grass can help reduce runoff and erosion, saving money in the long run.

Flooding in Cities

Flooding happens when rivers or drainage systems can't handle all the runoff. Floods are natural, but human changes to the land can make them worse. In cities, surface runoff is the main cause of urban flooding. This can damage property, contaminate water, destroy crops, and force people from their homes. Floods are very destructive natural disasters.

Stopping Runoff Problems

Runoff holding ponds

We can reduce the bad effects of runoff in several ways:

Smart City Planning: Design cities to have less paved area. This allows more water to soak into the ground.
Erosion Control: Use methods on farms and construction sites to stop soil from washing away.
Flood Control: Build things like holding ponds to store excess water during heavy rains.
Careful Chemical Use: Manage chemicals in farming and landscaping to prevent them from washing into waterways.

Smart City Planning

Many cities encourage building designs that let water soak into the ground. This includes using narrower sidewalks, special pavers for driveways, and other techniques to increase water absorption in urban areas.

Stopping Erosion

Farmers have used methods like contour farming for centuries to protect soil. Today, construction sites also use erosion controls. These include using straw bales and silt fences to slow down runoff on slopes. They also try to build during drier months and keep exposed soil covered.

Controlling Floods

Engineers have developed ways to predict and reduce flood peaks in rivers. Techniques include building holding ponds (also called detention basins) to store water temporarily. They also use special designs in channels to slow down water flow.

Managing Chemicals Safely

Laws like the U.S. Clean Water Act help control how toxic chemicals are stored and used. This prevents spills and leaks. Methods include requiring double-layered storage tanks and regulating fertilizers and pesticides. Industries often have to treat their waste before it goes into sewers.

The U.S. Clean Water Act also requires cities in urban areas to manage their stormwater systems. This means they must have programs to:

Educate people about preventing stormwater pollution.
Involve the public in local programs.
Find and remove illegal connections that dump pollution into storm drains.
Control runoff from construction sites.
Manage stormwater after construction is finished.
Prevent pollution (e.g., safe handling of fuels, fertilizers, pesticides) and keep systems clean.

Understanding Runoff with Science

Scientists use mathematical models and water sampling to study runoff. They can measure pollutants like chemicals, pH, and how cloudy the water is. They can also take water samples for testing.

Since the 1970s, computer models have helped analyze how runoff carries pollutants. These models consider how chemicals dissolve, how water soaks into soil, and how much pollution reaches rivers and lakes. These tools help us understand and plan ways to reduce runoff problems.