Student Features

Hydrologic Cycle

A bucket with a hole in the bottom illustrates how water recycles on earth by dripping in the top and out the bottom Many processes work together to keep Earth's water moving in a cycle. There are five processes at work in the hydrologic cycle: condensation, precipitation, infiltration, runoff, and evapotranspiration. These occur simultaneously and, except for precipitation, continuously.

Together, these five processes - condensation, precipitation, infiltration, runoff, and evapotranspiration- make up the Hydrologic Cycle. Water vapor condenses to form clouds, which result in precipitation when the conditions are suitable. Precipitation falls to the surface and infiltrates the soil or flows to the ocean as runoff. Surface water (e.g., lakes, streams, oceans, etc.), evaporates, returning moisture to the atmosphere, while plants return water to the atmosphere by transpiration.

Condensation is the process of water changing from a vapor to a liquid. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. The water vapor then has a change of state into liquid or ice.

Warm air rises, cool air condenses into clouds
Warm air rises, cool air condenses into clouds
You can see condensation in action whenever you take a cold soda from the refrigerator and set it in a room. Notice how the outside of the soda can "sweats?" The water doesn't come from inside the can, it comes from the water vapor in the air. As the air cools around the can water droplets form.

Precipitation is water being released from clouds as rain, sleet, snow, or hail. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air currents and falls.

Under some circumstances precipitation actually evaporates before it reaches the surface. More often, though, precipitation reaches the Earth's surface, adding to the surface water in streams and lakes, or infiltrating the soil to become groundwater.

A portion of the precipitation that reaches the Earth's surface seeps into the ground through the process called infiltration. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints), the more infiltration occurs. Water that doesn't infiltrate the soil flows on the surface as runoff.

A portion of the precipitation that reaches the Earth's surface seeps into the ground through the process called infiltration
A portion of the precipitation that reaches the Earth's surface seeps into the ground through the process called infiltration
Precipitation that reaches the surface of the Earth but does not infiltrate the soil is called runoff. Runoff can also come from melted snow and ice.

When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.

Evapotranspiration is water evaporating from the ground and transpiration by plants. Evapotranspiration is also the way water vapor re-enters the atmosphere.

Evaporation occurs when radiant energy from the sun heats water, causing the water molecules to become so active that some of them rise into the atmosphere as vapor.

Transpiration occurs when plants take in water through the roots and release it through the leaves, a process that can clean water by removing contaminants and pollution.

As you can see, many process are at work to give you the water you need. And these processes are always at work. Just because Antarctica is frozen doesn't mean that evaporation is not taking place (ice can turn directly to water vapor by a process called sublimation). And because the Sahara Desert is so dry doesn't mean that precipitation is not happening (it evaporates before it makes it to the ground).