The water cycle (hydrologic cycle) is the continuous movement of water on, above, and below Earth's surface through evaporation, condensation, precipitation, infiltration, and runoff. On the AP Enviro exam, it matters most where humans disrupt it, like impervious surfaces and saltwater intrusion.
The water cycle, also called the hydrologic cycle, is the nonstop loop that moves water between the atmosphere, the land, and bodies of water. The sun drives evaporation from oceans, lakes, and soil. That water vapor cools and condenses into clouds, falls back as precipitation, and then either soaks into the ground (infiltration, which recharges aquifers) or flows over the surface as runoff back toward streams and oceans. No water is created or destroyed in the cycle; it just keeps changing location and phase.
For AP Enviro, the cycle itself is the easy part. The exam cares about what happens when humans break a step in the loop. Per EK EIN-2.M.1, urbanization can deplete water resources and cause saltwater intrusion in the hydrologic cycle. And per EK EIN-2.M.3, impervious surfaces like roads, parking lots, and buildings block water from reaching the soil, so infiltration drops, runoff spikes, and cities flood. In other words, paving a city rewires the water cycle: water that should have recharged groundwater gets shunted into storm drains instead.
In Unit 5 (Land and Water Use), the water cycle is the backbone of Topic 5.10, Impacts of Urbanization. Learning objective 5.10.A asks you to describe the effects of urbanization on the environment, and two of the essential knowledge statements are really water-cycle disruptions in disguise. Pumping too much freshwater from coastal aquifers pulls salt water into the groundwater (saltwater intrusion, EK EIN-2.M.1), and impervious surfaces cut off infiltration and cause flooding (EK EIN-2.M.3). The cycle also feeds into Topic 5.11, since an ecological footprint compares a society's resource demands, and water is one of the biggest resources humans demand. If you can trace where a city interrupts the cycle, you can explain most urban water problems on the exam.
Keep studying AP Environmental Science Unit 5
Impervious Surfaces (Unit 5)
Roads, sidewalks, and parking lots are the water cycle's roadblock. They stop infiltration, so precipitation that should recharge soil and groundwater becomes surface runoff, which means urban flooding downstream and less water in aquifers.
Saltwater Intrusion (Unit 5)
When coastal cities pump groundwater faster than the cycle can recharge it, the freshwater pressure in the aquifer drops and seawater seeps in. This is the water cycle's recharge step losing a race against human demand.
Biogeochemical Cycles (Unit 1)
The water cycle is one of the biogeochemical cycles you learn in Unit 1, alongside carbon, nitrogen, and phosphorus. Unit 5 is where that Unit 1 knowledge gets applied, because urbanization questions expect you to already know how the unbroken cycle works.
Evaporation, Condensation, and Precipitation (Unit 1)
These three processes are the engine of the cycle. Knowing the order (evaporation up, condensation into clouds, precipitation down) lets you pinpoint exactly which step a human activity disrupts, which is what FRQ-style explanations reward.
Multiple-choice questions rarely ask you to recite the cycle. Instead they hand you a human disruption and ask you to name or explain it. Typical stems include identifying saltwater intrusion when coastal development over-extracts groundwater, and identifying impervious surfaces as the structures that prevent water percolation in urban areas. Data-analysis questions are common too, like a table of rising groundwater salinity (0.8 ppt in 1995 climbing to 3.1 ppt in 2020) paired with a 65% increase in urban water demand, where you connect over-pumping to intrusion. No released FRQ has used the phrase 'water cycle' verbatim, but the concept underpins urbanization FRQs constantly. Your job is cause-and-effect reasoning: name the step in the cycle that got blocked or overdrawn, then state the environmental consequence (flooding, aquifer depletion, salinization).
Both are biogeochemical cycles that urbanization disrupts, and Topic 5.10 covers them back to back, so it's easy to mix up which effect goes with which cycle. Urbanization hits the water cycle through resource depletion, saltwater intrusion, and impervious surfaces blocking infiltration (EK EIN-2.M.1 and M.3). It hits the carbon cycle through burning fossil fuels and landfills releasing CO2 (EK EIN-2.M.2). If the question is about flooding or groundwater, think water cycle. If it's about emissions, think carbon cycle.
The water cycle moves water continuously through evaporation, condensation, precipitation, infiltration, and runoff, powered by the sun.
Impervious surfaces like roads and parking lots block infiltration, which increases runoff and causes urban flooding (EK EIN-2.M.3).
Over-pumping groundwater in coastal cities causes saltwater intrusion, where seawater contaminates freshwater aquifers (EK EIN-2.M.1).
Infiltration is the step that recharges aquifers, so anything that blocks it (pavement) or outpaces it (over-extraction) creates water supply problems.
On the exam, water cycle questions are almost always about human disruptions, so identify which step of the cycle is broken and state the consequence.
Water demand is part of an ecological footprint, which measures the resources a person or society uses and the waste it produces.
It's the continuous movement of water between the atmosphere, land, and water bodies through evaporation, condensation, precipitation, infiltration, and runoff. On the AP exam it shows up most in Unit 5, where urbanization disrupts steps like infiltration and groundwater recharge.
Yes, they're the same thing. The CED uses 'hydrologic cycle' in EK EIN-2.M.1, so don't get thrown if an exam question uses that term instead of 'water cycle.'
Two main ways. Impervious surfaces (roads, buildings, parking lots) stop water from infiltrating the soil, which increases runoff and flooding. And heavy groundwater pumping depletes aquifers, which in coastal areas causes saltwater intrusion.
Urbanization disrupts the water cycle through saltwater intrusion and impervious surfaces blocking infiltration, while it disrupts the carbon cycle by burning fossil fuels and creating landfills that add CO2 to the atmosphere. Flooding and groundwater questions point to water; emissions questions point to carbon.
No. The cycle just moves and recycles the same water through different phases and locations. That's exactly why over-pumping aquifers is a problem: extraction can outpace the natural recharge rate, and the cycle can't magically replace what cities remove.