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Energy Moves
Water

Discover how solar energy and gravity work together to drive the continuous movement of water through Earth's systems.

Earth & Space Science

What Is the Water Cycle?

The water cycle (also called the hydrological cycle) is the continuous movement of water on, above, and below Earth's surface. Water changes state between liquid, gas, and solid as it moves through different reservoirs: oceans, atmosphere, glaciers, rivers, and underground.

Two Forces That Drive Water Movement

Solar Energy
The Sun heats water, providing the energy needed for evaporation and transpiration. It lifts water from surfaces into the atmosphere as water vapor.
Gravity
Gravity pulls water downward, driving precipitation, runoff, and infiltration. It moves water from high elevations back toward oceans and underground.

Key Vocabulary

Evaporation
Liquid water absorbs solar energy and changes to water vapor (gas).
Condensation
Water vapor cools and changes back to tiny liquid droplets, forming clouds.
Precipitation
Water falls from clouds as rain, snow, sleet, or hail due to gravity.
Transpiration
Plants release water vapor through tiny pores in their leaves.
Runoff
Water flows over land surfaces, pulled downhill by gravity to streams and oceans.
Infiltration
Water soaks into the ground, pulled downward by gravity into soil and rock.

The Water Cycle Model

Sun Cloud Ocean Groundwater Evaporation Transpiration Condensation Precipitation Runoff Infiltration

Energy Input: The Sun

The Sun is the primary energy source for the water cycle. When solar radiation reaches Earth's surface, it heats water in oceans, lakes, and soil. This thermal energy causes water molecules to move faster until they escape as vapor: a process called evaporation.

Solar Energy Liquid Water Evaporation Water Vapor Heat energy absorbed Molecules speed up and escape as gas

The Sun delivers about \(1,361 \text{ W/m}^2\) of energy to Earth's upper atmosphere

This is called the solar constant

The Role of Gravity

Once water is in the atmosphere, gravity takes over. Water droplets in clouds become heavy enough to fall as precipitation. On the ground, gravity pulls water downhill through rivers and streams (runoff) and downward into soil and rock (infiltration).

Snow Precipitation Runoff Infiltration Gravity River / Ocean

Solar Energy vs. Gravity

Solar Energy Gravity
Direction Moves water upward (into atmosphere) Pulls water downward (to surface)
Processes Evaporation, transpiration Precipitation, runoff, infiltration
State Change Liquid → Gas Gas → Liquid → falls (no state change needed)
Energy Type Thermal (heat) energy Gravitational potential → kinetic energy
Without It... No evaporation, cycle stops Water vapor never returns to surface

State Changes and Energy Transfer

Solid (Ice) +Energy Liquid (Water) +Energy Gas (Water Vapor) -Energy -Energy Melting Evaporation Freezing Condensation

How to Build a Water Cycle Model

1
Identify the reservoirs. Label where water is stored: oceans, atmosphere, glaciers, rivers, groundwater.
2
Draw the processes. Show arrows for evaporation, condensation, precipitation, runoff, infiltration, and transpiration.
3
Label the energy source. Mark solar energy as the driver for upward movement (evaporation, transpiration).
4
Label gravity. Mark gravity as the driver for downward movement (precipitation, runoff, infiltration).
5
Show state changes. Indicate where water changes between solid, liquid, and gas, and whether energy is absorbed or released.

Example 1: Tracing a Water Drop

A drop of water sits in the Pacific Ocean. Let's trace its journey through the water cycle and identify which force drives each step.

1
Solar energy heats the ocean surface. The water drop absorbs enough energy to evaporate into water vapor.
2
The water vapor rises and cools. It condenses onto a dust particle, forming a tiny cloud droplet. Energy is released.
3
The droplet grows heavy. Gravity pulls it down as precipitation (rain).
4
Gravity carries the drop as runoff down a hillside into a river, then back to the ocean.

Example 2: Mountain Snowmelt

Sun Snow Melting (+solar energy) Stream Gravity Lake

Example 3: The Rainforest Water Pump

In tropical rainforests, trees play a huge role. Their roots pull water from the soil, and leaves release it as vapor through transpiration. This creates a local cycle where water can fall as rain multiple times before reaching the ocean.

Soil (Groundwater) Transpiration Cloud forms Rain Root uptake Sun

Common Mistakes to Avoid

✕ "Water is created by rain"
Water is not created or destroyed. Precipitation moves existing water from the atmosphere to the surface.
✕ "Only the Sun drives the cycle"
Without gravity, water vapor would never return to the surface. Both forces are required.
✕ "Evaporation only happens in oceans"
Evaporation occurs from any water surface: lakes, puddles, soil, and even plant leaves (transpiration).

Summary: Processes at a Glance

Process Energy Source Direction State Change
Evaporation Solar Up Liquid → Gas
Transpiration Solar Up Liquid → Gas
Condensation Releases energy Stays in atmosphere Gas → Liquid
Precipitation Gravity Down None (liquid/solid falls)
Runoff Gravity Downhill None (liquid flows)
Infiltration Gravity Down None (liquid seeps)

Question 1 of 5

What is the primary energy source that drives evaporation in the water cycle?

Question 2 of 5

Which force is most responsible for pulling precipitation from clouds to Earth's surface?

Question 3 of 5

A student draws a water cycle model but forgets to include gravity. Which processes would be missing from the model?

Question 4 of 5

During transpiration, where does the water vapor come from?

Question 5 of 5

Snow on a mountaintop melts in spring and flows into a river. Which pair of energy sources is responsible for this sequence?