Hydrologic Cycle: Movement of Water

In this view of Earth from space, it is clear that water is an important part of our planet. Seventy percent of Earth's surface is covered by water, and water droplets in the form of clouds can be seen swirling above the surface. This water is in constant motion.

Another compartment is the freshwater compartment. It represents the earth's rivers, streams, and lakes and contains 0.16% of the earth's water.

A lot of the Earth's water is hidden from view under the ground. This groundwater compartment holds 0.68% of the earth's water.

An arrow points from freshwater to the ocean

The precipitation that falls on freshwater such as rivers and streams may immediately begin moving back to the ocean. The flow of streams and rivers is called runoff. Some of this runoff comes directly from precipitation, however, as we will see shortly, some comes from the land (overland flow) and some comes from groundwater (base flow).

Arrows point from land to freshwater and land to ocean

Some precipitation that falls on land runs over the ground and eventually reaches a feshwater body. This flow over the surface of the land without using streams and rivers is called overland flow. In coastal areas, water may flow to the ocean by overland flow. Note the cyclical nature of this system. Evaporation, condensation, precipitation, and flow create the cyclical pattern (see image at right).

Arrow moves downward in land compartment.

Not all water that lands on the ground finds its way to rivers and streams. Some of this water soaks into the soil through a process called infiltration. This region of soil moisture is called the zone of aeration or the vadose layer (vadose means shallow). This is the layer from which the roots of plants draw their water. Some of this water travels up the plant and evaporates into the atmosphere in a process called transpiration.

Arrow points downward from land to groundwater

Water, as it is pulled by gravity, percolates more deeply into the soil and may reach the water table where the ground is completely saturated with water (zone of saturation). This is groundwater. The upper boundary of the groundwater region is called the water table. If an area of groundwater is large enough, it's called an aquifer.

Arrows point from groundwater to freshwater and from groundwater to the oceanwater a

Groundwater can flow, but it flows very slowly. Its movement may be measured in feet per year. Groundwater can flow to a river or lake where it flows in unseen below the water body's surface. This is called baseflow. Groundwater may emerge at a spring. Groundwater can also flow into the ocean.

This whole system is powered by the energy of the sun which provides the energy for evaporation. When you see a river flowing down the side of a mountain, keep in mind that it was evaporation, powered by the sun, that lifted that water to the top of the mountain so that it could begin flowing down to the ocean again.

The atmosphere contains 0.002% of Earth's water in the form of water vapor, water droplets, and ice crystals.

Arrows point upward to atmosphere from land, freshwater, and ocean.

Evaporation (the changing of water from a liquid to a gas) is occurring constantly. Water evaporates from the ocean, from freshwater, and from the land. Most of that evaporation is from oceans, because that's where most of the water is, and more evaporation takes place in warmer climates than in cooler climates because heat increases the rate of evaporation. Some evaporation takes place from the surfaces of plants. This process is called transpiration.

Arrows point downward from the atmosphere to land, freshwater, and the ocean

As water from evaporation rises into the atmosphere, it cools and condenses (changes back to a liquid). The atmosphere returns water to the land, freshwater, and oceans in the form of precipitation (e.g., rain, snow, sleet, hail) Most precipitation falls on the ocean, because 70% of the earth's surface is ocean.

The land compartment holds 2.1% of the earth's water in the form of ice, snow, and depression storage. Depression storage is the storage of water in small depressions such as puddles. Some depression storage water will evaporate, and some will infiltrate into the soil.

We can think of all of the earth's water as being contained in five compartments. Water can move within a compartment and from one compartment to another.

Click Image to Enlarge