4.1THE HYDROLOGIC CYCLE

Have You Ever Wondered …

Why rivers continue to flow?

Where rivers even get their water?

What makes it rain?

How much water covers the Earth?

The Water CycleThe water cycle is the continuous

movement of water on Earth’s surface from the atmosphere to the land and oceans and back to the atmosphere.Water is essential for humans and all other

organisms.

The water cycle describes the existence and movement of water on, in, and above the Earth.Water covers 71% of Earth’s surface.Driven by energy from the sun.

The Water Cycle – EvaporationEvaporation is the process in which liquid water from the oceans and

Earth’s surface changes from a liquid into a gas (water vapor).

Water vapor is gaining energy.

About 86% of all evaporation is from the oceans, while the remaining 14% is from inland and vegetation. Evaporation Animation

The Water Cycle – EvaporationTranspiration is the process by

which plants release water vapor into the atmosphere.

Evapotranspiration is the total loss of water from an area, equaling the sum of water lost from evaporation and transpiration.

The Water Cycle – CondensationCondensation is the process in which water

vapor in the air is changed into liquid water. Responsible for cloud formation.

Water vapor is losing energy.

Warm air can hold MORE water vapor than cool air.

Molecules of water are becoming more organized going a gas to a liquid state.

Recall, the level of the disorder in molecules is called entropy. Liquid water to water vapor (evaporation) entropy increasing

Water vapor to liquid water (condensation) entropy decreasing

The Water Cycle – PrecipitationAny form of water falling from the clouds in the form

of rain, freezing rain, sleet, snow, or hail is known as precipitation.

The Water Cycle – PrecipitationIt is the primary connection in the

water cycle providing the delivery of atmospheric water to the Earth.

Most falls in the form of rain.

Precipitation varies in amount, intensity, and form by season and geographic location.

Precipitation does not fall in the same amounts throughout the world, a country, or even a city.

The Water Cycle – InfiltrationInfiltration is the entry of water into the soil

surface.

Infiltration results in the growth of vegetation and helps to sustain the continuous ground water supply to wells, springs, and streams.

The rate of infiltration is influenced by the following factors:

Physical characteristics of the soilSoil cover (i.e. plants, vegetation)Water content of the soilSoil temperature Rainfall intensity

The Water Cycle – PercolationPercolation is the downward movement

of water through soil and rock. Occurs beneath the root zone.

Ground water percolates through soil similar to water filling a spongeWater moves from space to space along

fractures in rock, through sand and gravel, or through channels in formations such as cavernous limestone.

The terms infiltration and percolation are often used interchangeably.

The Water Cycle – RunoffRunoff is the movement of water,

usually from precipitation, across the Earth's surface towards stream channels, lakes, oceans, or depressions in the Earth's surface.

The characteristics that affect the rate of runoff include:

Rainfall durationRainfall intensity Elevation (slope)Soil type

Vegetation