The Water Cycle
Every drop of water on Earth has been recycled countless times. The water in a glass today may have fallen as rain on a dinosaur, spent thousands of years frozen in a glacier, or evaporated from the Pacific Ocean last week.
THE WATER CYCLE is driven by energy from the Sun (which powers evaporation) and the force of gravity (which pulls water downward as precipitation, runoff, and infiltration). It has no beginning and no end.
The Six Main Processes
Evaporation: liquid water absorbs solar energy and changes to water vapor. Occurs mainly from ocean surfaces (about 86% of global evaporation) but also from lakes, rivers, soil, and puddles. Rate increases with temperature, wind speed, and low humidity.Transpiration: plants absorb liquid water through their roots, transport it through stems and leaves, and release water vapor through tiny pores called stomata. A large oak tree can transpire over 150,000 liters per year. Evaporation and transpiration together are called evapotranspiration.Condensation: water vapor cools (typically by rising and expanding) until it reaches the dew point, then forms tiny liquid droplets or ice crystals on condensation nuclei (dust, pollen, sea salt). This is how clouds and fog form.Precipitation: when cloud droplets or ice crystals grow large enough, gravity pulls them down as rain, snow, sleet, or hail. This is the primary way water returns from the atmosphere to the surface.Runoff: water that falls on the surface and does not soak in flows downhill under gravity, first as sheet flow, then collecting into rills, streams, and rivers that carry it to lakes or the ocean.Infiltration: water that soaks into the ground through soil and rock. Some is taken up by plant roots (returning to the cycle via transpiration). Some percolates deeper, recharging groundwater aquifers below the water table.Reservoirs and Residence Time
A reservoir is any place where water is stored: oceans, ice caps, groundwater, lakes, rivers, the atmosphere, and living organisms.
Residence time is the average length of time a water molecule spends in a reservoir before moving on:
Oceans: ~3,200 yearsGlaciers: decades to hundreds of thousands of yearsGroundwater: weeks to tens of thousands of yearsRivers: ~2 weeksAtmosphere: ~9 daysShort residence time means water cycles through quickly. Long residence time means water is stored for extended periods, which matters because water that enters a deep aquifer may not return to the surface for thousands of years.
What Drives the Cycle
Energy from the Sun: powers evaporation and transpiration, lifting water from the surface into the atmosphere. Without solar heating, the cycle would stop.The force of gravity: pulls precipitation to the ground, drives runoff downhill, and pulls infiltrated water deeper into aquifers.The cycle is continuous and global, at any moment, water is evaporating, condensing, falling, running off, and soaking into the ground simultaneously, everywhere on Earth.Human Impacts on the Water Cycle
Urbanization: paved surfaces block infiltration, increasing runoff volume and speed, causing more frequent flooding and reducing groundwater recharge.Deforestation: removing trees eliminates transpiration, reducing water recycled back to the atmosphere, and increases erosion and runoff.Irrigation: withdrawing large volumes of groundwater or river water for agriculture moves water between reservoirs faster than natural processes would.Climate change: warming increases evaporation rates, shifts precipitation patterns, and accelerates ice melt, intensifying wet events while worsening dry seasons, with regional variation.Key Vocabulary
Evaporation: Liquid water changing to water vaporTranspiration: Water vapor released by plants through leavesCondensation: Water vapor cooling into liquid droplets or ice crystalsPrecipitation: Water falling from clouds as rain, snow, sleet, or hailRunoff: Water flowing downhill across the surfaceInfiltration: Water soaking into the groundReservoir: Any place where water is storedResidence time: Average time a water molecule spends in a reservoirEvapotranspiration: Combined evaporation and transpiration