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What is Runoff? - Definition & Effects

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  • 0:01 What Is Runoff?
  • 0:27 What Affects Runoff?
  • 1:57 Runoff Effects
  • 3:10 Lesson Summary
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Lesson Transcript
Instructor: Jeff Fennell

Jeff has a master's in engineering and has taught Earth science both domestically and internationally.

Groundwater, surface water, and evaporated water are three important parts of the water cycle. In this lesson, we will learn about runoff and what causes it. We will also explore its impact on the environment.

What Is Runoff?

Runoff can be described as the part of the water cycle that flows over land as surface water instead of being absorbed into groundwater or evaporating. According to the U.S. Geological Survey (USGS), runoff is that part of the precipitation, snow melt, or irrigation water that appears in uncontrolled surface streams, rivers, drains, or sewers.

What Affects Runoff?

There are a variety of factors that affect runoff. Some of those include:

Amount of Rainfall

The amount of rainfall directly affects the amount of runoff. As expected, if more rainfall hits the ground, more rainfall will turn into runoff. The same can be said about snowmelt. If a large amount of snow melts in a short time period, there will be a large amount of runoff.

Permeability

The ability of the ground surface to absorb water will affect how much surface runoff occurs. If you have ever poured water onto sand, you may have noticed it sinks into the sand almost instantaneously. On the other hand, if you pour water on the street, the water will not sink but runoff to the gutter or a ditch. The less water the ground can absorb, the more runoff on the surface there will be. This is called permeability.

A surface with high absorption ability has high permeability, and a surface with low absorption ability has low permeability. This image shows permeability rates for different types of surfaces (see video).

Vegetation

Vegetation needs water to survive, and a plant's root system is designed to absorb water from the soil. There is less runoff in highly vegetated areas because the water is used by the plants instead of flowing off the surface of the ground.

Slope

The slope of a surface is also important to the amount of runoff there will be. The steeper a surface is, the faster it will flow down the slope. A flat surface will allow the water time to absorb.

Runoff Effects

While runoff is affected by various things like amount of rainfall and vegetation, too much of it can have a bad effect on the environment as well. Some examples include erosion and pollution.

Erosion

As runoff progresses, it can collect things that are in its way, transport them and drop them off somewhere downstream when the water slows down. You may have noticed things floating in rivers or streams. Moving water is a strong force that is able to move these objects. Small runoff is able to move lightweight items such as leaves and pebbles, whereas large runoff, like a flash flood, is able to wash away cars and even houses.

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