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Unit 1•Lesson 6 2 min read

Hydrology and the Water Cycle

6/18

Learning Objectives

Describe the major processes of the water cycle.
Define a watershed (or drainage basin).
Distinguish between porosity and permeability.
Describe the zones of groundwater, including the water table.

The Continuous Movement of Water

The water cycle (or hydrologic cycle) is the continuous movement of water on, above, and below the surface of the Earth.

Key Processes:

Evaporation: The process by which water changes from a liquid to a gas or vapor. The primary source of energy is the sun.
Transpiration: The process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere.
Condensation: The process by which water vapor in the air is changed into liquid water, forming clouds.
Precipitation: Water released from clouds in the form of rain, freezing rain, sleet, snow, or hail.
Infiltration: The process by which water on the ground surface enters the soil.
Runoff: The flow of water that occurs when excess water from rain, meltwater, or other sources flows over the land.

Watersheds

A watershed, also known as a drainage basin or catchment area, is any area of land where precipitation collects and drains off into a common outlet, such as into a river, bay, or other body of water. Small watersheds for creeks and streams are nested within larger watersheds for rivers.

Groundwater

Groundwater is the water present beneath Earth's surface in soil pore spaces and in the fractures of rock formations.

Porosity: A measure of the void (empty) spaces in a material. It is a fraction of the volume of voids over the total volume. A rock can be porous but not permeable.
Permeability: A measure of the ability of a material (such as rock) to allow fluids to pass through it. It depends on the connectivity of the pores. Sandstone is both porous and permeable, making it a good aquifer (a body of permeable rock which can contain or transmit groundwater). Clay may be porous but has very low permeability.

Zones of Groundwater:

Zone of Aeration (Unsaturated Zone): The region above the water table where the pores in the soil and rock are filled with both air and water.
Water Table: The upper surface of the Zone of Saturation. The water table depth can vary with seasons and precipitation.
Zone of Saturation: The region below the water table where all pore spaces are completely filled with water.

Key Terms

Water Cycle
The cycle of processes by which water circulates between the Earth's oceans, atmosphere, and land, involving precipitation, drainage, evaporation, and transpiration.
Watershed
An area or ridge of land that separates waters flowing to different rivers, basins, or seas.
Groundwater
Water held underground in the soil or in pores and crevices in rock.
Porosity
The quality of being porous, or full of tiny holes. It is the ratio of the volume of voids to the total volume of a material.
Permeability
The state or quality of a material or membrane that causes it to allow liquids or gases to pass through it.

Check Your Understanding

1

What is the difference between porosity and permeability?

2

Name and describe the three major processes that move water from the Earth's surface/oceans into the atmosphere.

3

What is the water table?

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