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Earth Science
Water Cycle
How Water Moves Through Earth's Systems
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The water cycle describes how water moves through the ocean, atmosphere, land, living things, and underground layers. It matters because every cloud, river, glacier, aquifer, crop, and drinking water supply depends on this continuous circulation. Solar energy and gravity drive most of the movement, while plants, soils, rocks, and people shape where water is stored and how fast it moves. Understanding the water cycle helps explain weather, climate, floods, droughts, erosion, and water availability.
Key Facts
- Evaporation rate increases when temperature, wind speed, and surface area increase.
- Condensation occurs when water vapor cools to its dew point and forms liquid droplets or ice crystals.
- Water balance can be written as P = R + I + ET + ΔS, where P is precipitation, R is runoff, I is infiltration, ET is evapotranspiration, and ΔS is change in storage.
- Stream discharge is Q = A v, where Q is discharge, A is cross-sectional area, and v is flow velocity.
- Groundwater moves from areas of higher hydraulic head to areas of lower hydraulic head.
- Transpiration is water vapor released by plants, and evapotranspiration = evaporation + transpiration.
Vocabulary
- Evaporation
- Evaporation is the process in which liquid water changes into water vapor and enters the atmosphere.
- Condensation
- Condensation is the process in which water vapor cools and changes into liquid droplets or ice crystals.
- Precipitation
- Precipitation is water that falls from the atmosphere to Earth's surface as rain, snow, sleet, or hail.
- Infiltration
- Infiltration is the movement of water from the land surface into soil and porous rock.
- Aquifer
- An aquifer is an underground layer of permeable rock or sediment that stores and transmits groundwater.
Common Mistakes to Avoid
- Thinking the water cycle is a simple circle, because real water can follow many pathways and stay in storage for seconds, years, or thousands of years.
- Confusing evaporation with boiling, because evaporation can happen at temperatures below boiling when molecules escape from a liquid surface.
- Assuming all rain becomes river runoff, because some precipitation infiltrates soil, is taken up by plants, evaporates, or becomes groundwater.
- Ignoring human water use, because pumping groundwater, building dams, paving land, and irrigating crops can change flow paths and water storage.
Practice Questions
- 1 A storm drops 40 mm of rain on a watershed. If 12 mm becomes runoff, 18 mm infiltrates, and 6 mm returns to the atmosphere by evapotranspiration, what is the change in water storage ΔS?
- 2 A river channel has a cross-sectional area of 25 m² and an average water velocity of 1.6 m/s. Use Q = A v to find the river discharge in m³/s.
- 3 A city replaces forest and soil with roads, parking lots, and rooftops. Explain how this change affects infiltration, runoff, groundwater recharge, and flood risk.