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Runoff & Infiltration: Definition & Process

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  • 0:03 Runoff and Infiltration
  • 1:39 Rate of Infiltration
  • 4:11 Effects of Runoff
  • 6:55 Lesson Summary
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Lesson Transcript
Instructor: Sarah Friedl

Sarah has two Master's, one in Zoology and one in GIS, a Bachelor's in Biology, and has taught college level Physical Science and Biology.

When water falls to the ground as precipitation, it can either fill the spaces in the soil below ground or run across the surface. In this video lesson, you will learn about the processes of infiltration and runoff, and how they are an important part of the water cycle on Earth.

Runoff and Infiltration

Water on Earth is constantly cycling through different phases and locations. Remember reading those Choose Your Own Adventure books as a kid? You got to a point in the story where your decision dictated what happened next. There were different options, and your path through the story line was directly tied to the twists and turns at those decision points.

Water is found naturally in all three phases on Earth: solid (glaciers and polar ice caps), liquid (oceans, lakes, and streams), and gas (water vapor in the atmosphere). As it moves through the water cycle on Earth, which is the natural circulation of water between surface water, air, and ground, it faces choices, like going through a Choose Your Own Adventure book. Water in the atmosphere can either stay in the atmosphere or it can fall back to Earth as precipitation. Likewise, the water in oceans, lakes, and streams has the option to evaporate into the atmosphere, which is when water turns from liquid to gas.

When precipitation hits the ground, it has two options depending on where it is in its story line. It may infiltrate the soil or continue across the land as runoff. Infiltration occurs when surface water enters the soil. This process is similar to pouring water onto a sponge. The sponge soaks up the water until it can hold no more. At this point, the soil becomes saturated, but the excess water has to go somewhere. When this happens, we get overflow in the form of runoff, which is when surface water flows over land.

Rate of Infiltration

Just like how fast a sponge can absorb water depends on the characteristics of the sponge itself, infiltration into the soil depends on the characteristics of that soil. As mentioned before, the amount of water the soil can hold is a major factor, and this is called the porosity of a soil. Porosity is all about the size, shape, and packing of soil particles. When there are particles of various sizes and shapes, we tend to have less porous soils. Think of a jar of mixed nuts. The smaller nuts fill in all of the spaces between the larger nuts, so we would say this is less porous because there is less open space. However, if you have a can of just large Brazil nuts, the spaces in between would be larger, so it would be more porous.

Clay is a type of soil that is actually quite porous. This is because the particles in clay are flat and packed in such a way that they don't line up very well and leave lots of open space. Sand is also very porous because the particles are all about the same size, so they also leave lots of open space between them.

Porosity tells us about the amount of water that can infiltrate, but not how fast or how well it does so. Permeability describes the rate of infiltration into the ground by water. It's like this: if you pour water over a sponge, it will absorb the water because it is very permeable. Try doing the same thing over a rock, and it will run right off like water off a duck's back. This is because the rock is not very permeable, but the sponge is.

Some soils are both porous and permeable, and some are porous, but not very permeable at all. Remember how I said that clay is very porous? There is a lot of potential there, because there's lots of space for water. But, it's not very permeable at all, so water doesn't infiltrate it well.

Limestone is interesting because it literally is a rock sponge - it is very permeable, but not very porous at all! Most of the time, if you pour water over a rock, it will run right off. However, limestone is so permeable that it if you pour water onto it, the water is soaked up into the limestone. This is why the world's aquifers are made up of mostly limestone. Like giant sponges underground, they hold and store water for long periods of time.

Effects of Runoff

Once the ground has been infiltrated to a point where it can't hold any more water, we get runoff. Just like continuing to pour water into an already full cup, the excess water has to go somewhere. With the full cup, it runs over the top onto the table. On the ground, the water runs over the surface of the land. This is an important part of the water cycle because it helps the water travel to different locations or even go through phase changes (from liquid to gas or solid).

However, water is a very powerful force, and as it moves across land, it leaves its mark everywhere it goes. As water travels, it picks up nutrients and sediments, like rocks, sand, and gravel. When these particles, whether large or small, are carried along by water, they act like sandpaper scraping across the land. This process of soil being removed from the surface of the land is known as erosion. After a while, the particles are deposited somewhere away from their original location, and this process of soil particles being deposited elsewhere is known as deposition. The Grand Canyon is one example of erosion on a very large scale. It's hard to believe that the waters of the Colorado River are responsible for carving out this unique and amazing feature!

Another issue with runoff is that, along with soil particles, any chemicals or pollutants that are present on the land may also be transported by the moving water. Think back to the full cup of water that you continue to pour water into. As the excess water runs across the table, it will pick up any dirt, food, or other things that are there. The same thing happens with runoff on the ground's surface. Runoff usually ends up in rivers, lakes, streams, and oceans, which provide valuable drinking water for us. So, when runoff carries pollutants and chemicals to these locations, they impact both environmental and human health.

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