1.7 The Hydrologic Cycle

The hydrologic cycle is the sun-powered movement of water between reservoirs as it shifts among solid, liquid, and gas. In AP Environmental Science, you should be able to explain how water leaves Earth's surface through evaporation and transpiration, returns as precipitation, and then runs off, percolates, or gets taken up by plants.

Why This Matters for the AP Environmental Science Exam

The water cycle is one of the biogeochemical cycles you need to explain step by step, including how water moves between sources and sinks. On the exam, you may be asked to describe the individual steps of the cycle using a diagram, identify reservoirs, and predict what happens when one part of the cycle changes. A common challenge is identifying the reservoir, which is the part that holds water the longest, so being able to name oceans, ice caps, and groundwater as reservoirs is useful. You should also be ready to connect human activities, like clearing forests or building dams, to disruptions in the cycle and the effects on people and ecosystems.

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Key Takeaways

• The hydrologic cycle is powered by the sun and moves water through its solid, liquid, and gas phases between sources and sinks.
• Oceans are the primary reservoir of water at Earth's surface; ice caps and groundwater are much smaller reservoirs.
• Water enters the atmosphere through evaporation (from water surfaces) and transpiration (from plants); together these are called evapotranspiration.
• Water returns to Earth as precipitation (rain, snow, sleet, hail).
• After reaching the surface, water can run off into bodies of water, percolate into groundwater, or be taken up by plants.
• Human activities such as deforestation, dam building, agriculture, urbanization, and pollution can disrupt the cycle and harm ecosystems and people.

The Water Cycle

The water cycle, also called the hydrologic cycle, is one of the biogeochemical cycles in AP Environmental Science. Water often carries other matter and chemicals to where they need to go, so it connects to the carbon, nitrogen, and phosphorus cycles. At the same time, water has its own cycle driven by energy from the sun.

The whole cycle is powered by the sun. Solar energy heats water, drives evaporation, and keeps water moving between reservoirs in its solid, liquid, and gaseous phases.

Water Leaving the Surface

Evapotranspiration describes all the water that leaves Earth's surface and enters the atmosphere as water vapor. It includes two processes:

• Evaporation: Heat from the sun turns liquid water at the surface into water vapor that rises into the atmosphere.
• Transpiration: Plants release water vapor through their leaves.

Water Returning to the Surface

Once water vapor is in the atmosphere, it eventually comes back down as precipitation (rain, snow, sleet, or hail). After precipitation reaches the ground, three things can happen:

• Surface runoff: Water flows across the land back into a body of water. This does not have to be immediate. Snow can sit on a mountain for months before it melts and runs off. Runoff helps keep water balanced in reservoirs.
• Percolation: Water soaks into the ground and becomes part of the Earth's groundwater. Soil absorbs the water and stores it below the surface. Groundwater can sometimes connect with nearby water sources.
• Plant uptake: Plants take in water and use it for photosynthesis. This can happen directly or after water has been absorbed by soil or groundwater.

Reservoirs

A reservoir is where water is stored, and the main reservoir matters on the exam. The oceans are the primary reservoir of water at Earth's surface. Ice caps and groundwater act as much smaller reservoirs. Knowing which reservoir holds the most water, and which step holds water the longest, helps you answer reservoir questions accurately.

Human Impacts on the Water Cycle

Humans can disturb the water cycle in several ways:

• Deforestation: Clear cutting a forest removes the trees that would take up water. This can lead to flooding and soil erosion.
• Pollution and climate change: Human activity can pollute water and shift climate patterns, which depletes and contaminates the water supply. Disruptions can also make erosion and heat waves more extreme.
• Agriculture and urbanization: Watering crops and growing, concentrated cities increase the demand for water.
• Diversion structures: Dams and pipes redirect water. This can disrupt the natural cycle and harm wildlife that depend on the water.

These are applications of the concept, showing how changing one part of the cycle affects people and ecosystems.

How to Use This on the AP Environmental Science Exam

MCQ

Expect questions that ask you to read a water cycle diagram and identify steps like evaporation, transpiration, precipitation, runoff, and percolation. You may also be asked which reservoir holds the most water (oceans) or which part of the cycle stores water the longest.

Free Response

If you need to explain the cycle, describe each step in order rather than just naming it. Practice walking through how water moves from a reservoir into the atmosphere and back to the surface. When a question changes one part of the cycle (for example, removing forest cover), predict the downstream effects on flooding, erosion, groundwater, or water availability for ecosystems and humans.

Common Trap

Naming a step is not the same as describing it. If a prompt says "describe," explain what happens during evaporation or percolation, not just the word. Also be careful to connect a change to a clear cause-and-effect outcome, since vague answers lose points.

Common Misconceptions

• The sun does not power the cycle. The hydrologic cycle is powered by the sun. Solar energy drives evaporation and keeps water moving.
• Ice caps or groundwater hold most of Earth's surface water. Oceans are the primary reservoir. Ice caps and groundwater are much smaller reservoirs.
• Evaporation and transpiration are the same thing. Evaporation is water leaving surfaces like oceans and lakes. Transpiration is water released by plants. Together they make up evapotranspiration.
• Precipitation is only rain. Precipitation also includes snow, sleet, and hail.
• Runoff returns to water instantly. Runoff can be delayed. Snow may stay frozen on a mountain for months before melting and flowing into a body of water.

Related AP Environmental Science Guides

• 1.1 Introduction to Ecosystems
• Unit 1 Overview: The Living World: Ecosystems
• 1.2 Terrestrial Biomes
• 1.4 The Carbon Cycle
• 1.5 The Nitrogen Cycle
• 1.3 Aquatic Biomes