This video, we're going to talk about the first biogeochemical cycle in our lesson, which is the water cycle. And you can see the diagram of the water cycle here behind me. Now what's important to recall is that water is essential to all life on Earth as we know it. And the water cycle is important because it helps to ensure that water is actually available in all ecosystems, including not only aquatic ecosystems but terrestrial ecosystems too. Without the water cycle, this would not be the case, and life on Earth would not exist as we know it.
So the water cycle is incredibly important. That being said, let's go ahead and wipe this clean so that we can approach this one step at a time. Because 97% of all of Earth's water is in the oceans, this makes the oceans the largest reservoir of water on Earth. We'll start our explanation of the water cycle here. So what you should know is that over the ocean, there's going to be net evaporation of water, and that evaporated water can rise up into the atmosphere, cool, and then condense to form rain clouds like the one that you can see here.
Now, of course, some of this water can precipitate back down into the ocean. But once again, it's important to know that evaporation rates over the ocean exceed precipitation rates over the ocean, which is why this red evaporation arrow is so much larger than this blue precipitation arrow. Now the atmosphere is also going to be a reservoir of water, and these rain clouds that are formed can be moved by winds over the land, where evapotranspiration from the land can contribute to the formation of these rain clouds. Now the term evapotranspiration is a single term that includes your standard evaporation from lakes and rivers, for example, but it also includes the process of transpiration, which recalls the process of plants releasing water into the atmosphere. Now over the land, there's going to be net precipitation, which is opposite of what we said over the ocean.
And so this net precipitation blue arrow is larger over land than the evapotranspiration red arrow is over land. Now once liquid water has precipitated onto the surface of the land, this makes water available to terrestrial organisms, and water can actually run off into the oceans from streams and rivers, for example, and that completes the water cycle. But also some of the water on the surface of the land can actually percolate downwards through the soil and collect in groundwater, such as aquifers for example. And then there can be underground runoff of water back into the oceans, again completing the water cycle. So this here completes our lesson on the water cycle, and we'll be able to apply these concepts and problems.
So I'll see you in our next video.