Water Cycle

The Water Cycle

Origins of Water

Geologists believe that liquid water has existed on Earth from very early geologic time. How did it get here? No humans were here when water first formed so no one knows for sure. But there are many theories. A few are listed below.

  • Water is stored in the magma of the Earth's core and was released by greatly increased volcanic activity in the distant past.

  • Water came to Earth from extra-terrestrial sources in the form of water-laden comets.

The Water Cycle or Hydrologic Cycle

The overall volume of water on the Earth can be considered constant even though small amounts are being added via volcanic activity.

Most of the water on the Earth has been here since it originally formed. Water that is part of you (humans are 50-80% water) could have once been part of a dinosaur.

So, the amount of water on Earth stays relatively constant but it certainly changes state, moves around a lot, and is stored in "temporary storage units". This process is known as The Water Cycle or The Hydrologic Cycle and consists of 4 main parts.

  • EVAPORATION: State change from liquid to gas.

  • CONDENSATION: State change from gas to liquid.

  • PRECIPITATION: Interaction of liquid with gravity.

  • COLLECTION: Interaction of liquid and gravity.

Below is an illustration of the Water Cycle from the United States Geologic Survey. The Water Cycle is driven by the energy of the Sun. It has no beginning and no end. The cycle is dependent on the fact that water exists in all 3 states at temperatures normally found on the Earth. Lets examine parts of the Hydrologic Cycle more closely.


The oceans comprise approximately 97% of the water on the Earth; by far the largest storage unit of liquid water. The Sun heats the oceans surface and some water evaporates into the air as water vapor.

Glaciers, ice and snow are another way that water is stored; sometimes for thousands of years. Solid ice and snow can change directly from the solid state to gaseous water vapor in a process called sublimation.

Water molecules held in the soil evaporate into the air as water vapor and plants release water from their leaves in a process called transpiration. These combined processes are known as evapotranspiration.

A large oak tree can transpire 40,000 gallons of water per year and plants, like cacti, that live in dry desert climates have physical adaptations that permit them to transpire much less water than other plants.

Evaporation from oceans, seas, lakes and rivers accounts for 90% of evaporated water vapor in the atmosphere. The other 10% is from plant transpiration.

A water molecule spends, on average, approximately 10 days in the atmosphere before condensing into a liquid form. This is why long range weather prediction is nearly useless.


It is estimated that the atmosphere stores approximately 3,100 cubic miles of water at any given time. This sounds like a lot but actually is only .001% of the Earth's total water volume.

This gaseous water vapor eventually transforms back to the liquid state in a process known as condensation. We are all familiar with condensation; water beading on the outside of a glass of ice water, fog, fogging glasses and condensation on our windows on a cold day.

Atmospheric air containing water vapor rises and cools. This cooling causes the water vapor to condense to the liquid state and clouds are formed. The air higher in the atmosphere is cooler than that at the surface of the Earth. Why? Mainly because there is less of it at higher altitudes. This means less collisions of molecules due to a given amount of solar energy, and therefore, a lower temperature.

Water droplets continue to combine, or coalesce, into larger and larger water droplets. This coalescense often involves tiny particles of dust, salt, smoke, or pollutants such as SO2.


Precipitation is the interaction between water droplets in clouds with the Earth's gravity. Water droplets form and coalesce until they become too heavy to remain in the atmosphere. They then fall to the Earth's surface in the form of rain, freezing rain, sleet, snow, or hail.

Precipitation does not fall in the same amounts throughout the world and the amount of precipitation is not evenly spaced throughout the year. Animals and plants have life cycles that allow them to adapt to these differences in precipitation. In western Pennsylvania, on average, we receive 37.85 inches of rain per year. As you can see from the graph this rainfall is distributed relatively evenly throughout the year. In what month do we usually receive the least rain? The most?

Average annual rainfall graph for Pittsburgh, Pennsylvania


Gravity continues to influence precipitation once it reaches the Earth's surface. Rain flows downhill overland from high to low elevations when it encounters saturated or impervious soils. Some of it eventually finds its way back to one of the temporary "storage units" (ocean,glacier,groundwater). Along the way, this "universal solvent" will pick up minerals, nutrients, soil particles and pollutants that it encounters.

Surface runoff is responsible for a good deal of stream flow but only about one-third of precipitation falling on land finds its way to an ocean. The other two-thirds is evaporated, transpired, or soaked into the ground as groundwater. It is often also diverted by humans for our uses.

Infiltration is the process by which precipitation moves downward from the land surface into soil and porous rock. This subsurface water forms saturated and unsaturated zones. In the unsaturated zone the spaces between soil particles contain both air and water. Below the unsaturated zone water fills all of these spaces or voids. This is the saturated zone or groundwater zone. The water table is the interface between the saturated and unsaturated zones.

Groundwater and surface waters are constantly interacting. Groundwater provides the low flow or base flow of permanent streams. Have you ever wondered how streams continue to flow even when it hasn't rained in many days?

A spring is groundwater that finds its way to the surface and some streams actually flow underground into caves and become groundwater.

Many of the old mines in western Pennsylvania have become filled with water. This water often becomes acidic due to minerals that it picks up while in the mine. During heavy rain events these acidic groundwaters are flushed from the mine and enter our surface streams; including Peters Creek.