The Hydrologic Cycle (College Board AP® Environmental Science): Study Guide

What is the hydrologic cycle?

• The hydrologic cycle can also be called the water cycle

• The hydrologic cycle describes the continuous movement of water in different forms (solid, liquid, and gas) through Earth's systems

• The hydrologic cycle is powered by the sun

  • The sun drives processes like evaporation and precipitation

• Water moves between sources (where it is released) and sinks (where it is stored or absorbed)

Awaiting image: Hydrologic cycle
Image caption: A hydrologic cycle diagram showing the key water transfers and reservoirs

Key steps in the hydrologic cycle

• The hydrologic cycle is continuous and interconnected, as water constantly moves between the atmosphere, land, and water bodies

• Each step depends on the others to maintain Earth's water balance

1. Evaporation from water bodies:

   • The Sun heats water in oceans, lakes, and rivers, causing it to turn into water vapor and rise into the atmosphere

   • This process is called evaporation and is the main way water enters the atmosphere

2. Transpiration from plants:

   • Plants also release water vapor into the atmosphere through their leaves in a process called transpiration

   • Together, evaporation and transpiration are often referred to as evapotranspiration

3. Condensation in the atmosphere

   • Once in the atmosphere, the water vapor cools as it rises, turning into tiny droplets or ice crystals

   • These droplets gather to form clouds in a process called condensation

4. Precipitation back to the surface

   • When clouds become too heavy with water droplets, the water falls back to the Earth's surface as precipitation (rain, snow, sleet, or hail)

5. Infiltration into the ground

   • Some of the precipitation soaks into the ground in a process called infiltration, replenishing underground aquifers

6. Runoff into water bodies

   • Water that doesn’t infiltrate flows over the land as runoff, collecting in rivers, lakes, and eventually returning to the oceans

7. Groundwater flow and discharge

   • Water stored underground in aquifers slowly moves and eventually discharges into rivers, lakes, or oceans, completing the cycle

Hydrologic reservoirs

• Hydrologic reservoirs are places where water is stored during the cycle

• Water moves between these reservoirs through processes like evaporation, precipitation, infiltration, and runoff

Key hydrologic reservoirs

1. Oceans:

   • The largest reservoir, holding about 97% of Earth's water

   • Supplies most of the water that evaporates into the atmosphere

2. Ice caps and glaciers:

   • Hold about 2% of Earth's water, mainly in frozen form

   • Serve as long-term storage reservoirs for freshwater

3. Groundwater:

   • Stores water beneath the Earth's surface in aquifers

   • Provides a source of drinking water and irrigation

4. Atmosphere:

   • Holds water vapor, which contributes to weather patterns and precipitation

   • Water spends only a short time here before returning to the surface

5. Lakes, rivers, and oil:

   • Act as smaller, temporary reservoirs

   • Play a key role in surface water flow and local ecosystems

Reservoir interactions in the hydrologic cycle

• Evaporation:

  • The process by which liquid water changes into a gaseous state (water vapor) and enters the atmosphere from water bodies such as oceans, lakes, and rivers

• Transpiration:

  • The process by which plants absorb water from the soil through their roots and release it as water vapor through tiny openings called stomata in their leaves

• Evapotranspiration:

  • The combined processes of water vaporization from the Earth's surface (evaporation) and the release of water vapor by plants through transpiration

• Sublimation:

  • The direct transition of water from a solid (ice or snow) to a vapor state without melting first

• Condensation:

  • The process in which water vapor in the atmosphere transforms into liquid water, forming clouds or dew, as a result of cooling

• Advection:

  • The horizontal movement of water vapor, clouds, or precipitation caused by the prevailing wind patterns

• Precipitation:

  • The process of water falling from the atmosphere to the Earth's surface in the form of rain, snow, sleet, or hail

• Melting:

  • The process by which solid ice or snow changes into liquid water due to an increase in temperature

• Freezing:

  • The process by which liquid water changes into a solid state (ice or snow) due to a decrease in temperature

• Flooding:

  • The overflow of water onto normally dry land, often caused by heavy rainfall, melting snow, or dam failure

• Surface runoff:

  • The movement of water over the Earth's surface typically occurs when the ground is saturated or impermeable, leading to excess water.

• Infiltration:

  • The process of water seeping into the soil from the surface, entering the soil layers and becoming groundwater

• Percolation:

  • The downward movement of water through the soil and underlying rock layers, eventually reaching aquifers or groundwater reservoirs

• Groundwater flow:

  • The movement of groundwater through soil and rock layers within an aquifer, driven by gravity and pressure differences

• Stream flow or channel flow:

  • The movement of water in streams, rivers, or other water bodies, driven by gravity and the slope of the land, ultimately leading to oceans or lakes