1.7 The Hydrologic (Water) Cycle

Water Moves Through Reservoirs

The hydrologic cycle describes water's continuous movement among reservoirs: oceans (largest reservoir), atmosphere, glaciers/ice caps, groundwater, and surface water. Solar energy drives the cycle by powering evaporation (liquid to gas) and transpiration (water loss from plants), together called evapotranspiration.

Key Processes

Condensation forms clouds as water vapor cools; precipitation (rain, snow) returns water to Earth's surface. Infiltration allows water to seep into soil, recharging aquifers (groundwater), while runoff carries water over land into streams and rivers, eventually reaching oceans via surface flow.

Human Impacts on the Cycle

Deforestation reduces transpiration and increases runoff, causing erosion and flooding. Urbanization creates impervious surfaces (pavement) that increase runoff and decrease infiltration, lowering groundwater recharge. Overpumping aquifers depletes groundwater faster than natural recharge, causing saltwater intrusion and land subsidence.

Key terms

Hydrologic cycle

The continuous movement of water among reservoirs like oceans, atmosphere, groundwater, and surface water, driven by solar energy.

Evapotranspiration

The combined process of evaporation (from surfaces) and transpiration (from plants) releasing water vapor into the atmosphere.

Infiltration

The process by which water seeps into soil and recharges groundwater aquifers.

Runoff

Water flowing over land into streams and rivers; increased by impervious surfaces like pavement.

Aquifer

An underground layer of permeable rock or sediment that stores and transmits groundwater.

Try an AP-style question

What is the largest reservoir of water on Earth?

1. The oceans
2. Groundwater
3. Glaciers and ice caps
4. The atmosphere