What is the Hydrologic Cycle?
The hydrologic, or water, cycle refers to water movement on, above, and within the earth. Now, during the water cycle, or hydrologic cycle, water can be in any one of its three states during different phases of the cycle.
During all the different phases of the cycle, water is going to be in the form of liquid water, solid ice or snow, and water vapor (which is water’s gas form). You’ve got solid, liquid, and gas. All three states of water can be found in different phases of the hydrologic cycle.
Let’s look at all the different processes that can be involved in the hydrologic cycle. Precipitation is when water vapor falls to the earth as rain, snow, sleet, or hail. It’s coming from the atmosphere down to the earth.
When water’s falling from the atmosphere to the earth, that is precipitation. Rain is the most common form you’ll see, but if it’s colder, you can have snow, sleet, or hail. Next, when precipitation lands on plant foliage instead of the ground.
Water is falling, but there’s so much foliage that the water is going to fall on the plants instead of the ground. This is known as canopy interception. The canopy, which would be the top of the foliage layer, is intercepting the water.
Runoff produced by melting snow is known as “snow melt”. If you have big drifts of snow and they start to melt, the water is going to runoff downhill and join any lakes, streams, or rivers nearby.
Sometimes, when snow starts to melt in the spring, there will be flooding, because rivers that weren’t used to having so much water are going to have all this excess water from snowmelt. Next, when water flows from the surface into the ground, that is known as infiltration.
The water is infiltrating the ground. Water that flows underground. Once the water has infiltrated and is flowing underground in your groundwater, that is called subsurface flow (below the surface). When liquid water changes to a gas, that is evaporation.
When solid water, such as snow or ice, changes to a gas without going through the liquid phase (it goes straight from ice or snow to water vapor), then that is known as sublimation. Think about if you dropped an ice cube into a skillet that’s been on the stove that is really hot.
You’re going to see a lot of steam come up. The ice didn’t turn to liquid first. It went straight to steam, because there was such a drastic temperature change. That is a process known as sublimation.
The movement of water through the atmosphere is known as advection. When water vapor changes to liquid water, that’s known as condensation. If there’s water vapor in the air and it condenses into liquid water, that is the process of condensation.
When water vapor is released from plants into the air, that is known as transpiration. Sometimes water falls on the plants, the plants take it in, and they also take in water through their roots from the ground. Whenever they release water through their leaves, that is known as transpiration.
Let’s look at a picture and kind of look at an example of each of these. The most common things you’re going to see in your water cycle are, well, let’s just say we’ve got water. This is your liquid water. Let’s say we’ve got some land on the side of this river.
We’ve got our liquid water. We’ve also got groundwater underneath our land. You’ve got your water just standing on the ground. From there, water will evaporate and go through the atmosphere. When it goes through the atmosphere, you’ve got advection as it is evaporating.
You’ve got advection as it’s moving through the atmosphere and evaporating it’s changing from water to gas (water vapor). Once it gets up into the air, it’s going to keep going and condense. When the water vapor that’s in the air condenses into tiny particles, it forms clouds.
That is your condensation. When you form these clouds, that is water vapor condensing onto tiny particles in the air and joining together. When the clouds get heavy enough, they’re going to release that water.
It’s going to have advection, because you’re going to be having water move through the atmosphere again, but it’s going to be falling as precipitation. The most common things you’re going to see are evaporation, condensation, and precipitation.
Let’s look at the other ones, too. You’ve got rain falling down and some of it’s going to fall into the water. That’s fine. If it falls onto the ground, then we’re going to have infiltration. Whenever we’ve got this groundwater, there’s going to be subsurface flow. Let’s see what we’re missing.
We’ve got our precipitation falling down. Canopy interception. We’ve got our rain falling over here and we’ve got some little flowers and we’ve got some big trees. When the rain falls on this tree, we have a canopy interception.
The water is being intercepted by the canopy of the tree. Snow melt. Let’s go this way with it. Let’s say we’ve got kind of a hill going on here and you’ve got some mounds of snow. It’s going to start melting and coming down the hill and draining in here.
This is going to be your snow melt. This is the runoff from that snow melt. We discussed infiltration, subsurface flow and your groundwater, evaporation, sublimation (when solid water is going to change to gas without going through a liquid phase).
You may see some of that here if it gets warm enough and the snow just directly evaporates into the air without actually melting into the liquid form. Some of it may just evaporate into the air. Any that is coming up off the snow, that would be sublimation.
We got evaporation. Advection we talked about. If it’s moving through the atmosphere, that’s your advection. Condensation we’ve got (falling to the earth), and transpiration. Then, when these plants give off water into the atmosphere, you have transpiration.
That covers all the different processes that could be involved in the hydrologic cycle. There are lots of them. The most important ones you want to remember are evaporation, condensation, and precipitation, because they’re the ones you’re going to see most often.
Every other one of these processes is involved in the hydrologic cycle, which is basically just explaining all the different ways that water can move in, on, and above the earth’s surface.