What Is The Hydrologic Cycle?

Water, water, every where,

Nor any drop to drink.

— Samuel Taylor Coleridge

The preceding passage from Coleridge’s “The Rime of the Ancient Mariner” about a sailor stranded at sea also pretty aptly describes the Earth’s abundance of water and lack of drinkable access to most of it. In fact, 93% of the world’s water is locked in oceans, and is toxic to humans and many plants and animals.

hydrologic cycle

The remaining unlocked water circulates through the Earth’s ecosystem through a process called the “hydrologic cycle.” This process is quickly defined as the movement of water through the hydrosphere, the region of Earth’s atmosphere and surface containing all water. The peculiar properties of water enable this process to happen.

In molecular form, water is two parts hydrogen and one part oxygen, hence “H2O.” Depending on the temperature, water exists in three primary forms: solid, liquid, and gas.

When liquid water heats up, the hydrogen bonds holding the water molecules together break and send the molecules into the atmosphere, creating gas.

When liquid water cools, the hydrogen bonds lose energy and become more rigid—creating ice.

It is thanks to this malleable nature that the hydrologic cycle, the process by which drinkable water circulates the Earth, is able to exist. This cycle is broken up into five parts: Condensation, Precipitation, Infiltration, Runoff, and Evaporation.

What is Condensation?

dew on leafAs noted earlier, when temperatures change, the fundamental structure of water molecules changes as well. So when gaseous water molecules in the atmosphere lose energy thanks to cooling temperatures, a change has to happen. In this case, condensation occurs.

Near the Earth’s surface, water molecules undergoing this same process cling to other solid objects. The most recognizable form of condensation? The dew that decorates plant-life (among other things) in the morning.

Up in the troposphere—the lowest layer of Earth’s atmosphere—water molecules undergoing the same process cling to other particles hovering about in the air, such as dust, salt, or smoke particles, creating clouds of all different shapes and sizes.

One of the major forms of hydrological transportation is the wind. As these clouds amass more airborne water molecules, local wind currents push them rather vast distances. Once the air becomes too saturated with water vapor, however, a different type of transportation happens.

What is Precipitation?

rain in the mountainsAs the water vapor molecules continue colliding, these individual droplets get increasingly more massive. Eventually, the updrafts that keep clouds afloat are no longer able to support the water weight held in these clouds. At this point, atmospheric condensation turns into precipitation.

Precipitation, in a nutshell, is any form of water that pelts your head on the one day you forget to carry an umbrella. Depending on weather conditions, the primary forms of precipitation are drizzle, rain, sleet, snow, and hail. Temperature and pre-existing precipitation play a factor in what specific type forms.

These types of precipitation also carry with them whatever non-water molecules they bonded with up in the troposphere. Gas pollution, often caused by manmade items such as factories, power plants, and motor vehicles, contaminates the air and is often a base material for condensation. This produces highly acidic precipitation, which is known as acid rain.

Once precipitation, acidic or not, hits the Earth’s surface, three separate processes occur: Infiltration, Runoff, and Evaporation.

What is Infiltration?

water overflow on farm fieldIf liquid water gets caught between a rock and hard place, it doesn’t get stuck in a philosophical dilemma; it enters the soil.

Whenever it rains, snows, or otherwise precipitates, a portion of liquid water seeps into the soil or rock that makes up the Earth’s surface. How much, if any, water successful enters the terrain depends on a number of factors, including soil permeability, saturation, land slope, and land cover.

Denser soil types, like clay, are less permeable and do not allow much infiltration. Looser soil types, like sand, are the opposite.

Much like the clouds before precipitation occurs, soil can also only be saturated with water to a certain point. After a torrential rainstorm, even the most permeable of soils reach a point where they can not accept any more precipitation. The rest turns into a runoff, another major aspect of the hydrologic cycle.

Another factor is land slope. Water on a flat slope pools into stagnant puddles, which allows for more time to soak into the earth; water on a steep incline is more likely to become runoff.

Lastly, heavy land cover, such as rocks, trees, and other vegetation, is likely to trap precipitation and make it infiltrate, whereas smooth surfaces, like parking lots or sidewalks, pour the water elsewhere.

Infiltrated groundwater moves both horizontally and vertically through the soil, and often deposits into underground aquifers or back into a stream or ocean.

What is Runoff?

water overflow runoff from fieldIf liquid water gets caught between a rock and a hard place and a parking lot (or any other number of factors), then it is likely to run off and become runoff, the next major element of the hydrologic cycle.

Anything that forbids infiltration creates surface runoff. Less permeable soil, saturated soil, and steep sloping land generate more runoff than infiltration. The water cannot enter the earth, so it follows gravitational forces and is displaced elsewhere.

This often manifests in the form of water rivulets that eventually meet to combine larger channels of water, such as creeks, streams, and rivers. From here, the water usually deposits in ponds, lakes, oceans, or underground basins.

As time passes and the Earth grows more populated, an increase in man-made constructs has caused an increase in potentially potent runoff. Impenetrable surfaces (like parking lots, roads, or sidewalks) and cleared vegetation contribute to increased runoff, which often leads to flooding in urban areas.

No matter where runoff water collects, it is eventually subject to the fifth element of the hydrologic cycle: evaporation.

What is Evaporation?

water evaporates When liquid water tires of being chained down to Earth by virtue of gravity, it enlists in the help of the sun.

Thanks to the sunlight (or some other nearby heat source), energy causes the temperature of the water molecules to rise and for hydrogen bonds connecting the molecules to break. The warm air rises, carrying the water vapor, and brings it back into the atmosphere.

Without evaporation, the Earth’s already miniscule percentage of viable drinking water would be trapped in the ponds, lakes, oceans, or other forms of relatively still water. As 70% percent of the Earth’s surface is covered in oceans, evaporation occurs at an enormous scale across the globe every day.

The process also causes water molecules to abandon whatever detritus has been attached to the water during its precipitation, infiltration, or runoff phases. Seawater, once evaporated, leaves behind salt and other minerals, as those molecules are no longer carried through the atmosphere by the weight of liquid water droplets.

After evaporation, water molecules begin to condense, starting the entire hydrologic cycle over again.

Water for Thought

  • If the hydrologic cycle stopped, what would happen to societies? To the planet?
  • What would happen to water from melting polar ice caps?
  • Where would people go as water levels continued to rise?
  • Can the ocean water replace freshwater if it disappears? How could people adjust to make oceans a source of consumable water?

For More Information


Photo sources:
Water cycle CC-BY-SA-3.0 or GFDL, via Wikimedia Commons
Condensation: ZeePack / Foter / CC BY-ND 2.0
Precipitation: timsackton / Foter / CC BY-SA 2.0
Infiltration: Alternative Heat / Foter / CC BY 2.0
Runoff: eutrophication&hypoxia / Foter / CC BY 2.0
Evaporation: Budgemont / Foter / CC BY 2.0