AP Environmental Science

## Learning Outcomes

- Understand how **solar energy** and **gravity** work together to drive the hydrologic cycle
- Identify the difference between **evaporation**, **transpiration**, and **evapotranspiration**
- Explain how water moves from the atmosphere back to Earth through condensation and precipitation
- Distinguish between **runoff**, **infiltration**, and **percolation**
- Predict how vegetation, deforestation, and urbanization change water movement
- Apply hydrologic-cycle reasoning to AP Environmental Science multiple-choice and free-response questions

## AP Environmental Science Syllabus

For the AP Environmental Science, you are expected to understand sun-powered water movement within the hydrologic cycle, with a focus on the following syllabus points:

- Know that the hydrologic cycle is driven primarily by **solar energy** and **gravity**
- Identify the sequence **evaporation/transpiration → condensation → precipitation → runoff/infiltration/percolation**
- Explain the difference between **evaporation**, **transpiration**, and **evapotranspiration**
- Understand that **oceans are the largest water reservoir** and the main source of global evaporation
- Distinguish between **infiltration** and **percolation/groundwater recharge**
- Predict how **vegetation**, **deforestation**, and **impervious surfaces** affect runoff, infiltration, and evapotranspiration
- Recognise that water cycle processes connect the atmosphere, hydrosphere, geosphere, and biosphere
- Understand that water storage can occur in surface water, groundwater, snow, ice, soil, and living organisms

## Test Your Knowledge

_Attempt these questions before reading this article. If you find some difficult or cannot remember the answers, look more closely at that area during your revision._

1. Why is the hydrologic cycle described as sun-powered if gravity is also essential?
2. What is the difference between evaporation, transpiration, and evapotranspiration?
3. How does water get from the atmosphere into groundwater?
4. Why does urbanization usually increase runoff and decrease infiltration?
5. How would cutting down trees change local water movement?

## Introduction

The hydrologic cycle is the continuous movement of water through the atmosphere, land, oceans, groundwater, and living organisms. For AP Environmental Science, this is not just a diagram to memorise. You need to understand **what causes each step**, how the steps connect, and how human activity changes the balance.

A common mistake is to say that the Sun drives the whole water cycle by itself. That is only partly true. The **Sun powers the upward movement** of water into the atmosphere, mainly by evaporation and transpiration. **Gravity drives the downward and downhill movement** of water back to Earth, through precipitation, runoff, infiltration, and groundwater flow. If you separate those two roles clearly, many APES questions become much easier.

> **Key Term: hydrologic cycle**
> The continuous movement of water among the atmosphere, land, oceans, groundwater, and living organisms.

## Introduction to the system

Water links all Earth systems at once. It moves through the **atmosphere** as water vapor and clouds, through the **hydrosphere** as oceans, lakes, rivers, and groundwater, through the **geosphere** as soil moisture and percolating water, and through the **biosphere** inside plants and animals.

The oceans are the biggest water reservoir on Earth, holding about **97%** of all water. Freshwater makes up only about **3%**, and much of that is not easy to use because it is trapped in glaciers and ice caps or stored underground. This matters for the exam because the water cycle is not mainly about “usable water.” It is about movement between reservoirs and pathways.

> **Key Term: reservoir**
> A place where a large amount of a substance is stored for a relatively long time.

A cloud, by contrast, is more like temporary storage. Water may stay there for only days before moving on. This helps explain why the water cycle is dynamic: water is always shifting location, form, and availability.

### Solar energy: why water moves upward

The first big idea is that liquid water must gain energy to enter the atmosphere. Solar radiation warms oceans, lakes, rivers, wet soil, and plant surfaces. When water molecules gain enough energy, they escape from the liquid phase and become water vapor.

> **Key Term: evaporation**
> The phase change in which liquid water absorbs energy and becomes water vapor.

This is why evaporation is strongest where there is abundant water and enough heat. Oceans dominate global evaporation because they cover most of Earth’s surface and receive large amounts of solar energy. They are also a major climate regulator. Water requires a lot of energy to evaporate, so evaporation removes heat from surfaces and helps prevent extreme warming.

Warm air can hold more water vapor than cold air. That fact explains a lot of hydrology and weather. If air temperature rises, evaporation often increases. That does not guarantee immediate rainfall in the same place, but it does increase the amount of water entering the atmosphere.

Plants add another major pathway.

> **Key Term: transpiration**
> The release of water vapor from plants, mainly through stomata in leaves.

Plants take up water from the soil through their roots, move it upward through their tissues, and release part of it into the atmosphere. This means vegetation is not just “using” water. It is actively returning water to the air. Forests, grasslands, and crops all help drive local and regional moisture movement.

When you combine evaporation from land surfaces with transpiration from plants, you get a term APES likes to test.

> **Key Term: evapotranspiration**
> The combined transfer of water to the atmosphere by evaporation from surfaces and transpiration from plants.

Evapotranspiration is not a third independent process. It is the **sum** of evaporation and transpiration. Students often lose marks by treating it as something separate.

A useful comparison is this:

- **Evaporation**: water leaves oceans, lakes, rivers, puddles, and soil
- **Transpiration**: water leaves plants
- **Evapotranspiration**: the total of both from land and vegetation

This distinction matters in land-use questions. A forested area usually has higher transpiration than a paved area. A parking lot may get very hot, but it has no leaves, roots, or stomata, so its water movement is limited mainly to surface evaporation after rain.

> **Test Tip:** If an APES question asks why forests return more water to the atmosphere than urban land, the strongest answer usually includes **transpiration** or **evapotranspiration**, not just “heat.”

There are a few other phase changes you may see in some diagrams. **Sublimation** is when ice changes directly to water vapor, and **deposition** is when water vapor changes directly to ice. These are real parts of the water cycle, especially in cold regions, but the core APES focus here is the Sun driving evaporation and transpiration.

### Condensation and precipitation: how atmospheric water returns

Once water vapor is in the atmosphere, cooling becomes the key step. Water vapor does not stay invisible forever. When air cools enough, water changes phase again.

> **Key Term: condensation**
> The phase change in which water vapor cools and becomes liquid water.

Condensation forms cloud droplets. But a cloud is not the same thing as precipitation. That distinction is easy to miss. A cloud forms when water vapor condenses, but precipitation only occurs when droplets or ice crystals grow large enough to fall under gravity.

> **Key Term: precipitation**
> Water that falls from the atmosphere to Earth as rain, snow, sleet, or hail.

This is where gravity becomes central. The Sun got the water into the atmosphere; gravity brings it back down. If air cools, condenses, and forms clouds, but the droplets stay too small, there is no precipitation yet.

Condensation also helps explain why changes in temperature affect rainfall patterns. Warmer air can hold more water vapor, so when that warm moist air later cools, it may produce heavy precipitation. This is one reason an intensified water cycle can increase the risk of extreme rainfall.

The simple pathway is:

1. Solar energy heats water
2. Water evaporates or transpires
3. Water vapor rises and cools
4. Condensation forms clouds
5. Precipitation returns water to Earth

That sequence is basic, but AP questions often ask you to explain **why** one step follows another. The answer usually depends on phase changes and energy transfer.

### Gravity at the surface: runoff, infiltration, and groundwater recharge

After precipitation reaches Earth, several pathways are possible. Some water flows across the land surface. Some enters the soil. Some gets taken up by plants and later transpired. Some becomes groundwater.

> **Key Term: runoff**
> Water that flows over the land surface into streams, rivers, lakes, and oceans instead of soaking into the ground.

Runoff increases when water cannot infiltrate quickly enough. That may happen because rainfall is intense, the ground is already saturated, the soil is compacted, vegetation has been removed, or the surface is impervious, like concrete or asphalt.

> **Key Term: infiltration**
> The process by which water enters the soil from the ground surface.

Infiltration is only the first step into the ground. Students often stop there and call it groundwater recharge, but that is incomplete.

> **Key Term: percolation**
> The downward movement of water through soil and permeable rock toward groundwater.

Percolation is what carries infiltrated water deeper into the ground. If enough water moves downward and reaches the saturated zone, it helps replenish groundwater.

> **Key Term: groundwater recharge**
> The replenishment of groundwater when infiltrated water percolates downward into an aquifer.

So the sequence is:

- **Infiltration**: water enters soil
- **Percolation**: water moves downward through soil and rock
- **Recharge**: groundwater storage is replenished

This matters in FRQs. If a prompt asks how a wetland, permeable soil, or green infrastructure helps water supply, you should mention infiltration **and** percolation or recharge, not treat them as identical.

Another point students miss is the role of vegetation before water even reaches soil. Leaves and branches can catch rainfall temporarily. This is called interception. Some intercepted water evaporates directly back into the atmosphere, reducing the amount that becomes runoff.

### How land cover changes the water cycle

One of the most testable parts of this subtopic is how different surfaces alter water movement. APES often asks students to compare natural land cover with deforested or urbanized land.

A vegetated area, especially forest, usually has:

- more interception
- more infiltration
- more transpiration
- less runoff
- lower flood risk

An urban area with lots of impervious surfaces usually has:

- less infiltration
- less groundwater recharge
- less transpiration
- more runoff
- greater flood risk

Why? Because water cannot soak through pavement, and there are fewer plants returning water to the atmosphere. Instead of moving slowly through soil and vegetation, precipitation rushes into drains, streams, and rivers.

Deforestation changes the cycle in a similar direction, though not always as dramatically as paving. Removing trees reduces interception and transpiration. It also often increases erosion, because roots no longer hold soil in place. That means more sediment can be carried into streams and lakes.

If a forested watershed is cut, you would expect:

- lower evapotranspiration
- reduced infiltration
- increased runoff
- greater soil erosion
- more variable stream flow
- higher flood risk after rain

This also connects to water quality. Runoff from cleared land may carry sediment, nitrates, phosphates, and other pollutants into water bodies. That can contribute to turbidity and eutrophication.

> **Test Tip:** In “predict” questions, always make the chain explicit: **less vegetation → less transpiration/interception → more runoff → more erosion/flooding**. AP readers reward clear cause-and-effect links.

Wetlands are especially useful in this context. Because they store water, slow its movement, and often enhance infiltration, they reduce runoff and flooding while helping recharge groundwater. Destroying wetlands removes those functions.

### Why the cycle is described as a dynamic equilibrium

The water cycle is often described as being in **dynamic equilibrium**. That means water is constantly moving, but on the global scale the overall cycle remains balanced over time: evaporation and transpiration return water to the atmosphere, while precipitation brings it back.

This does **not** mean every place receives the same amount of water it loses. Deserts, rainforests, coasts, and mountain regions are all very different. It means the global cycle continues through continuous exchanges among reservoirs.

It is also not a perfectly fixed system. Climate change can intensify or shift it. Warmer conditions can increase evaporation, alter precipitation timing, reduce snowpack, and lead to earlier snowmelt. So the cycle remains continuous, but the distribution and timing of water movement can change.

### Applying the idea in AP-style reasoning

When AP Environmental Science asks about the hydrologic cycle, the question is often less about naming every step and more about applying the process to a scenario.

For example:

- If a city replaces grass with asphalt, runoff rises and infiltration falls.
- If a forest is cut down, evapotranspiration falls and erosion rises.
- If a region warms, evaporation may increase and precipitation patterns may shift.
- If wetlands are restored, infiltration and water storage increase while flood risk decreases.

A strong answer explains both **mechanism** and **outcome**. Do not just say “urbanization is bad for the water cycle.” Say **how**: impervious surfaces reduce infiltration and recharge, increase runoff, and raise flood risk.

## Key Point Checklist

_This article has covered the following key knowledge points:_

- The hydrologic cycle is the continuous movement of water among atmosphere, land, oceans, groundwater, and living organisms
- The cycle is driven mainly by **solar energy** and **gravity**
- The Sun powers upward movement of water through **evaporation** and **transpiration**
- **Evapotranspiration** means evaporation plus transpiration
- Oceans are the largest water reservoir and the main source of global evaporation
- **Condensation** changes water vapor into liquid droplets and helps form clouds
- **Precipitation** returns water from the atmosphere to Earth as rain, snow, sleet, or hail
- **Runoff** is water flowing over land instead of entering the ground
- **Infiltration** is water entering soil, while **percolation** is the deeper downward movement through soil and rock
- Groundwater recharge occurs when infiltrated water percolates into aquifers
- Vegetation tends to increase interception, infiltration, and transpiration while reducing runoff
- Urbanization and deforestation usually increase runoff and flood risk while decreasing infiltration and groundwater recharge

## Key Terms and Concepts

- hydrologic cycle
- reservoir
- evaporation
- transpiration
- evapotranspiration
- condensation
- precipitation
- runoff
- infiltration
- percolation
- groundwater recharge


The Hydrologic (Water) Cycle - Sun-powered water movement