The Hydrologic
Water Cycle
Explore how water moves through Earth's systems via evaporation, condensation, precipitation, and runoff: the engine driving weather, climate, and life.
AP Environmental Science
What Is the Hydrologic Cycle?
The hydrologic cycle (water cycle) is the continuous movement of water within Earth and between the atmosphere, surface, and subsurface. It is driven by solar energy and gravity, and it has no true beginning or end.
Earth's Water Reservoirs
Oceans
~96.5% of all water. Saltwater drives global evaporation.
Ice & Glaciers
~1.74% of total. Largest freshwater store on Earth.
Groundwater
~1.69% of total. Stored in aquifers below the surface.
Surface & Atmosphere
<0.07% combined. Rivers, lakes, and atmospheric vapor.
Why It Matters
1
Climate regulation: Evaporation and precipitation redistribute heat energy across the globe, moderating temperatures.
2
Ecosystem support: Every terrestrial and aquatic ecosystem depends on the water cycle for nutrient transport and habitat maintenance.
3
Human resources: Agriculture, drinking water, industry, and energy production all draw from the water cycle.
4
AP Exam relevance: The water cycle connects to soil science, biogeochemical cycles, pollution transport, and climate change topics.
Core Processes of the Cycle
Water follows branching surface,soil,groundwater and atmospheric pathways
Infiltration crosses the land surface, but only part of infiltrated water becomes groundwater recharge. Water can remain in soil, enter roots, or move toward streams and the coast. Evaporation absorbs latent energy; condensation releases it.Branching hydrologic pathways
Infiltration crosses the land surface, but only part of infiltrated water becomes groundwater recharge. Water can remain in soil, enter roots, or move toward streams and the coast. Evaporation absorbs latent energy; condensation releases it.Six Key Processes
1
Evaporation: Solar energy converts liquid water to vapor from oceans, lakes, and soil surfaces.
2
Transpiration: Plants release water vapor through stomata: this is often grouped with evaporation as evapotranspiration.
3
Condensation: Water vapor cools and forms cloud droplets around condensation nuclei in the atmosphere.
4
Precipitation: Water falls as rain, snow, sleet, or hail when droplets grow heavy enough.
5
Runoff: Water flows over land surfaces into streams, rivers, and eventually the ocean.
6
Infiltration: Water seeps into soil and rock, recharging groundwater aquifers.
The Water Balance Equation
For any region over time, inputs must equal outputs plus change in storage:
\( P \)
Precipitation: all water input to the system
\( ET \)
Evapotranspiration: water returned to the atmosphere
\( R \)
Runoff: surface and subsurface flow leaving the area
\( \Delta S \)
Change in storage: groundwater, soil moisture, ice
Evaporation vs. Transpiration
| Evaporation | Transpiration | |
|---|---|---|
| Source | Open water, soil, wet surfaces | Plant stomata on leaves |
| Driver | Solar radiation, wind, humidity | Solar radiation, root uptake |
| Control | Physical (temperature, surface area) | Biological (stomatal regulation) |
| Global share | ~90% from oceans | ~10% of atmospheric moisture |
| Combined term | Evapotranspiration (\( ET \)) | |
Understanding Residence Time
Residence time is the average length of time a water molecule spends in a given reservoir. It is calculated as:
Atmosphere
~9 days
Rivers
~2 weeks
Oceans
~3,000 years
Glaciers
~10,000+ years
How to Trace a Water Molecule
AP questions often ask you to describe the path a water molecule takes. Use this systematic approach:
1
Identify the starting reservoir: ocean, lake, soil moisture, glacier, or groundwater.
2
Name the phase change: evaporation (liquid to gas), sublimation (solid to gas), or melting (solid to liquid).
3
Follow the atmospheric path: vapor rises, cools, and condenses onto nuclei forming clouds.
4
Describe the return: precipitation type, then runoff vs. infiltration vs. absorption by plants.
5
End at a reservoir: specify where the molecule is stored and roughly how long (residence time).
Example: Ocean to Rain
Example: Groundwater Recharge
Predict how groundwater withdrawal can affect surface water.
Use this illustration to explore the concept described in its caption.
Example: Water Balance Calculation
A watershed receives 1,200 mm of precipitation per year. Evapotranspiration removes 700 mm, and runoff carries away 400 mm. What is the change in storage?
Human Impacts on the Cycle
Urbanization
Impervious surfaces (roads, buildings) increase runoff and decrease infiltration.
Deforestation
Removes transpiration pathway, increases surface runoff and erosion.
Irrigation
Redirects surface and groundwater. Can deplete aquifers faster than recharge.
Common Mistakes
✕ Cycle has a "start"
The water cycle is continuous with no beginning or end. Never describe it as starting at one point.
✕ All precipitation becomes runoff
Precipitation partitions into runoff, infiltration, and evapotranspiration. The ratio depends on soil, slope, and land cover.
✕ Ignoring transpiration
Transpiration contributes ~10% of atmospheric moisture. On land, it can exceed physical evaporation. Always include it.
Summary of Processes
| Process | Phase Change | Direction | Energy |
|---|---|---|---|
| Evaporation | Liquid to Gas | Surface to Atmosphere | Absorbs (endothermic) |
| Transpiration | Liquid to Gas | Plant to Atmosphere | Absorbs (endothermic) |
| Condensation | Gas to Liquid | Atmosphere (in place) | Releases (exothermic) |
| Precipitation | None (already liquid/solid) | Atmosphere to Surface | Gravitational (PE to KE) |
| Infiltration | None | Surface to Subsurface | Gravitational |
| Runoff | None | Land to Ocean/Lake | Gravitational |
Question 1 of 5
Which process describes the release of water vapor from plant leaves into the atmosphere?
Transpiration is the biological process by which plants release water vapor through stomata. Evaporation occurs from non-living surfaces. Condensation is gas to liquid, and sublimation is solid to gas.
Question 2 of 5
A watershed receives \( P = 900 \, \text{mm/yr} \), loses \( ET = 500 \, \text{mm/yr} \), and has \( R = 350 \, \text{mm/yr} \). What is \( \Delta S \)?
Using \( P = ET + R + \Delta S \): \( 900 = 500 + 350 + \Delta S \), so \( \Delta S = +50 \, \text{mm/yr} \). The watershed is gaining stored water.
Question 3 of 5
Which reservoir has the longest average residence time for water?
Glaciers and ice caps have the longest residence time, often exceeding 10,000 years. This is because the flux (melting rate) is very low relative to the enormous volume of stored ice.
Question 4 of 5
A city replaces grassland with impervious concrete surfaces. Which change is most likely?
Impervious surfaces prevent water from infiltrating the soil, so runoff increases dramatically and infiltration decreases. This leads to higher flood risk and lower groundwater recharge.
Question 5 of 5
Condensation in the atmosphere releases energy. This energy was originally absorbed during which process?
Evaporation is endothermic: it absorbs latent heat to convert liquid water to vapor. When that vapor condenses, it releases the same latent heat back into the atmosphere. This is a key mechanism for heat redistribution in Earth's climate system.