How does water move between the atmosphere, land and oceans, and how do humans alter these flows?
Topic 1.7 The Hydrologic (Water) Cycle: describe the processes of the water cycle and explain how human activities alter the storage and movement of water.
A focused answer to APES Topic 1.7, covering evaporation, transpiration, condensation, precipitation, runoff, infiltration and groundwater, and how deforestation, paving and irrigation alter the cycle, with a worked water-budget calculation.
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What this topic is asking
The College Board (Topic 1.7) wants you to describe the water cycle (the hydrologic cycle): the processes that move water between the atmosphere, land surface, soil and oceans. You must also explain how human activities, especially deforestation, paving and water withdrawal, change where water is stored and how fast it moves.
The processes of the water cycle
The main processes are:
- Evaporation: liquid water (mostly from oceans) turns to vapor using solar energy.
- Transpiration: plants release water vapor from their leaves; combined with evaporation it is called evapotranspiration.
- Condensation: water vapor cools and forms clouds.
- Precipitation: water falls as rain, snow, sleet or hail.
- Runoff: water flows over the surface into streams, rivers and oceans.
- Infiltration: water soaks into soil and rock, recharging groundwater and aquifers.
Reservoirs
This is why human pressure on the small accessible-freshwater fraction matters so much.
How humans alter the cycle
Human activities change both the storage and the movement of water:
- Deforestation removes the trees that transpire and intercept rain, reducing evapotranspiration and infiltration and increasing runoff; it can reduce local rainfall.
- Paving and urbanization create impervious surfaces that block infiltration, so runoff and flooding increase while groundwater recharge falls.
- Irrigation and groundwater pumping withdraw water faster than aquifers recharge, lowering water tables and depleting aquifers.
- Dams and reservoirs store water and change the timing and location of flows downstream.
Because the water cycle connects every other system, these changes ripple outward. Less infiltration means less groundwater recharge and lower dry-season streamflow; more runoff means more erosion, more pollutant transport and higher flood peaks. Reducing transpiration by clearing forests can dry the local climate, while over-pumping aquifers that took thousands of years to fill removes water faster than it can be replaced. The water cycle is the carrier that links Unit 1's nutrient cycles to real landscapes: phosphorus and nitrogen reach waterways through runoff, and the same human changes that speed up runoff also speed up the delivery of those pollutants.
Try this
Q1. Identify the process by which water vapor forms clouds. [1 point]
- Cue. Condensation.
Q2. Explain why a city floods more easily after heavy rain than a nearby forest. [2 points]
- Cue. The city's impervious surfaces block infiltration, so almost all the rain becomes fast surface runoff, whereas the forest soil and roots allow much of the rain to soak in.
Exam-style practice questions
Practice questions written in the style of College Board exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AP 2022 (style)4 marksSection II (FRQ). A forest is cleared and replaced with a paved parking lot. (a) Identify the process by which water enters the atmosphere from plant leaves. (b) Describe how paving the surface changes infiltration and runoff. (c) Explain one consequence of increased runoff for local waterways. (d) Describe how removing the forest affects local precipitation patterns.Show worked answer →
A 4-point FRQ on human alteration of the water cycle.
(a) Identify (1 point): transpiration (water evaporating from plant leaves; evapotranspiration if combined with surface evaporation).
(b) Describe (1 point): an impervious paved surface prevents water soaking into the ground, so infiltration decreases and surface runoff increases sharply.
(c) Explain (1 point): increased runoff carries more water, sediment and pollutants quickly into waterways, raising flood risk and erosion and reducing water quality.
(d) Describe (1 point): removing trees reduces transpiration, lowering the moisture returned to the atmosphere locally, which can reduce local precipitation and dry the area.
Markers reward naming transpiration, linking paving to lower infiltration and higher runoff, a valid runoff consequence, and connecting deforestation to reduced transpiration and precipitation.
AP 2020 (style)1 marksSection I (multiple choice). Which process moves water from the soil into underground aquifers? (A) Transpiration (B) Infiltration (C) Condensation (D) Runoff. Justify your choice.Show worked answer →
A 1-point MCQ on water-cycle processes. The answer is (B).
Infiltration is the movement of water from the surface down into the soil and underlying rock, recharging groundwater and aquifers. Transpiration (A) releases water from plants to the air; condensation (C) forms clouds; runoff (D) flows across the surface into streams. The trap is confusing infiltration (downward into soil) with runoff (across the surface).
Related dot points
- Topic 1.4 The Carbon Cycle: describe the major reservoirs and fluxes of the carbon cycle and explain how natural processes and human activities move carbon between them.
A focused answer to APES Topic 1.4, covering carbon reservoirs and fluxes, photosynthesis and respiration, decomposition, combustion, the ocean as a carbon sink, and how fossil fuel burning alters the cycle, with a worked carbon-flux calculation.
- Topic 1.5 The Nitrogen Cycle: describe the steps of the nitrogen cycle and explain how nitrogen fixation, the role of bacteria and human activities move nitrogen between reservoirs.
A focused answer to APES Topic 1.5, covering nitrogen fixation, nitrification, assimilation, ammonification and denitrification, the central role of bacteria, and how synthetic fertilizer alters the cycle, with a worked nitrogen-input question.
- Topic 1.6 The Phosphorus Cycle: describe the phosphorus cycle, explain why it has no significant atmospheric component, and explain how phosphorus acts as a limiting nutrient and a pollutant.
A focused answer to APES Topic 1.6, covering the slow sedimentary phosphorus cycle, weathering and uptake, why there is no gas phase, phosphorus as a limiting nutrient, and how mining and detergents cause eutrophication, with a worked limiting-nutrient question.
- Topic 1.3 Aquatic Biomes: describe the major freshwater and marine biomes and explain how abiotic factors such as salinity, depth, light, temperature and nutrients shape them.
A focused answer to APES Topic 1.3, covering freshwater and marine biomes, salinity, the photic and aphotic zones, estuaries, coral reefs and wetlands, and the abiotic factors that control aquatic productivity, with a worked dissolved-oxygen question.
- Topic 1.2 Terrestrial Biomes: describe the global distribution of the major terrestrial biomes and explain how temperature and precipitation determine the type of biome found in a region.
A focused answer to APES Topic 1.2, covering the major terrestrial biomes, how temperature and precipitation define them, latitude and altitude patterns, and biome shifts under a changing climate, with a worked climograph question.
Sources & how we know this
- AP Environmental Science Course and Exam Description — College Board (2020)