The water cycle | Ecology | Khan Academy

Let's talk a little bit about the water cycle, which we're all familiar with. In fact, we're all part of the water cycle every moment of our lives. We might not fully appreciate it, so let's just jump into the cycle. I'll start with evaporation.

So, we could start with the surface of the ocean here, or this river, or this lake. At any given moment, there's going to be water evaporating off of that, off of that surface. Water molecules that were in their liquid state, they just have enough energy to bounce away and go into the gas state. Water in a gaseous state we call vapor—water vapor.

And so, that water vapor is going to rise, likely with the air that has been heated on the surface due to the sun. There are other more complex dynamics at play, but as it rises and as the overall temperature cools, that water vapor will condense into little droplets. It'll condense around little tiny air particles, little particles of dust that you can't even see with your eye, and that's what forms clouds.

So, these are little droplets; little droplets. The water is actually back to the liquid form. They're not individual water molecules anymore; they're now getting, they're now able to interact with each other, and they're condensing around these little microscopic dust particles to form these water droplets. If it's cold enough, they might also form small ice crystals, and that's what clouds are.

We see here they're talking about transportation; you can have these clouds. Obviously, if you look outside, you see clouds. Those clouds are moving with the wind, and so they could be moving all of those droplets with the overall wind. When those droplets get heavy enough, they will precipitate down. Now, they could precipitate back into where we started; they could go back to the ocean there, or you could go onto a mountain here.

And since if the air is cold enough and if you have the right conditions, that precipitation might be snow, and it might stay snow right over there or ice. But then eventually, things might warm up, or they might not warm up. But when they do warm up, well, then they would melt, and then they would be snowmelt runoff. That's what you're seeing there if that rain is falling in this area.

Let's say it's not cold enough for it to be snow, so we're talking about rain. Well, most of that water is actually going to percolate down into the soil. Most of it goes down. We look around us and we see these rivers and lakes, and we say, "Oh wow, there's a lot of water there." But it turns out there's actually a lot more water inside the ground and obviously in the ocean, and we'll talk about that in a little bit.

So, you have all this water that forms in these underground aquifers here, but some of it also ends up in these lakes. These lakes are usually in a situation where the ground is either already saturated with water or there's the right types of rocks so it can contain the water up here. Similarly, rivers are formed by runoff—snowmelt runoff can famously form rivers.

In general, if you see a creek or a river near your house, especially when it rains, it fills up. That's a good indication that the groundwater is already saturated, and so things are running off into that river. That, in general, is the water cycle.

You have evaporation; it condenses into clouds, it eventually precipitates, and it keeps going round and round and round. Now, of course, there are other actors at play. You have things like plants—plants will take up water from the upper soil as far as the plant's roots go, and it will use that water to transport nutrients down from the soil up into the leaves.

It also uses that water as part of the photosynthesis process that we've studied in many videos, and a lot of that water gets transpired—gets transpired out. So, once again, this is transpiration—essentially evaporation out of the leaves of the water. Over here, you see this word sublimation—that's going straight from the solid form of water (ice) into the gas form of water or water vapor.

This will happen in situations where it's cold, very dry, and you have, in general, low pressure. Instead of going into the liquid state, right then, the water molecules just start back, just leaving as water vapor. Now, of course, you know I said we're part of it. Well, how are we part of it?

Well, we drink some of this fresh water. Our bodies are actually mostly water, or the cells in our bodies are 70 percent water. Everything we study in biology—water is a key environment for all of these things to occur. Then we use that water, and then we will get that water out of our bodies, and then it continues on as part of this water cycle.

Now, one thing that I find really interesting as an organism dependent on fresh water—when people say fresh water, we're talking about water without salt as opposed to salt water. We really need this water that's in the fresh water in these lakes or in this river, or we might dig wells so that we can get the water out of these aquifers.

It actually turns out that very little of the overall water in the world is fresh water. So, let me show you this chart over here. I always knew that, but I didn't fully appreciate how little was fresh water.

Of all the water on our planet, 97.5 percent is salt water. For the most part, in our oceans, only two and a half percent is fresh water. Even of that two and a half percent fresh water, very little of it is what we traditionally associate fresh water—the lakes and the rivers.

When I think of fresh water, I mean, "Oh let me go to a lake or a river." That's the stuff that we could potentially drink. But most of it is actually in glaciers and permanent snow cover. So, it's ice, snow that is just not melting. It makes you think about what would happen if this stuff were to melt.

Then you also have groundwater, which we could have access to—that's why people dig wells. So, we're talking about well groundwater, which includes soil moisture, swamp water, and permafrost. But very little of the water is actually in lakes and rivers, which I personally find fascinating. It wasn't obvious to me before I frankly saw this chart.

Now, another really interesting thing is how long, on average, water molecules might stay in different parts of this water cycle. You know, going back here, you can imagine that a water molecule could stay for a very long time in the ocean. Especially, you know, it's going to be moving around depending on ocean currents and temperature and all of that.

But you could imagine it could stay in that liquid form in that ocean for a very long time, and maybe it spends a shorter amount of time in a cloud. People have actually studied this, which I find fascinating. I'd be curious to figure out how they actually got this data.

This is the average residence time for water molecules. You can see here that water can stay in glaciers and permafrost for a very long time. We're talking it could be up to 10,000 years, and these are all rough numbers. It could stay as groundwater anywhere from two weeks to ten thousand years, I guess depending on how isolated that groundwater is.

It could be in the oceans and seas as salt water for four thousand years. We can look at all of these all the way to, within living organisms, it'll stay about on average— a water molecule will last about a week in the atmosphere. That's getting water vapor turning into a cloud, precipitating down—on average, one and a half weeks.

Once again, these are averages; it doesn't mean that every water molecule will stay exactly one and a half weeks in the atmosphere, but it's a pretty interesting thing to think about. It gives you a little bit more sense of, well, one, where all the water is, and how it all works together with the water cycle.