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Water Cycle
Evaporation, Condensation, Precipitation, and Runoff
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The water cycle (hydrological cycle) describes the continuous movement of water through Earth's oceans, atmosphere, and land. Solar energy drives the cycle: it evaporates water from oceans and land surfaces, which rises as water vapor into the atmosphere. Transpiration from plants adds additional vapor. As water vapor rises and cools, it condenses around microscopic particles to form clouds. When droplets in clouds grow large enough, they fall as precipitation (rain, snow, sleet, or hail).
On land, precipitation follows three paths: it may run off over the surface into streams and rivers (surface runoff), be absorbed into the soil (infiltration), or evaporate back into the atmosphere. Infiltrated water may recharge groundwater aquifers or be taken up by plants. Glaciers store fresh water as ice for thousands of years before slowly releasing it. The water cycle is essential to distributing fresh water across the planet, moderating climate, shaping landscapes through erosion, and supporting all terrestrial and aquatic life.
Key Facts
- Evaporation + transpiration = evapotranspiration; solar energy drives water from surfaces into atmosphere
- Condensation: water vapor cools and changes to liquid around cloud condensation nuclei
- Precipitation: rain, snow, sleet, hail — returns water to Earth's surface
- Surface runoff flows to streams, rivers, lakes, and ultimately the ocean
- Infiltration: water soaks into soil → may reach groundwater aquifers
- The ocean holds ~97% of Earth's water; only 3% is fresh water, and ~70% of that is frozen in glaciers
Vocabulary
- Evaporation
- The process by which liquid water at the surface absorbs heat energy and becomes water vapor, entering the atmosphere.
- Transpiration
- The release of water vapor from plant leaves and stems through tiny pores (stomata); a major source of atmospheric moisture over land.
- Condensation
- The change of state from water vapor to liquid water, forming cloud droplets when air cools to the dew point temperature.
- Infiltration
- The movement of water from the surface into the soil and rock layers, replenishing groundwater and soil moisture.
- Aquifer
- An underground layer of permeable rock or sediment that holds groundwater, which can be extracted by wells.
Common Mistakes to Avoid
- Thinking clouds are made of water vapor. Clouds consist of tiny liquid water droplets or ice crystals suspended in the air. Water vapor is invisible; clouds become visible when vapor condenses.
- Assuming all precipitation reaches the ocean quickly. Water may be stored for years in soil, decades in groundwater, and millennia in glaciers before returning to the ocean.
- Confusing evaporation and boiling. Evaporation occurs at any temperature at the liquid surface when molecules gain enough energy to escape. Boiling occurs throughout the liquid at the boiling point.
- Forgetting that the water cycle interacts with human systems. Groundwater pumping, deforestation, and paving surfaces all alter infiltration and runoff rates, affecting local and regional water availability.
Practice Questions
- 1 Trace a single water molecule from the ocean through the full water cycle back to the ocean, naming each process and change of state.
- 2 Deforestation reduces transpiration in a region. How might this change local precipitation patterns? Explain the chain of effects.
- 3 Why is groundwater often considered a non-renewable resource on human timescales, even though it is part of the water cycle?