1.7 The Hydrologic Cycle

Overview

  • The hydrologic cycle is the continuous movement of water through Earth’s systems: atmosphere, land, surface water, groundwater, and living organisms.

  • It is driven by solar energy (evaporation, transpiration) and gravity (runoff, groundwater flow).

  • Water exists in three forms: liquid, solid (ice/snow), and gas (water vapor).

  • The cycle is essential for life, nutrient transport, ecosystem function, and climate regulation.

Major Water Reservoirs

  • Oceans: Largest reservoir; most evaporation comes from here.

  • Ice caps and glaciers: Hold freshwater long-term; melting contributes to runoff.

  • Groundwater (aquifers): Water stored in porous rock underground; slowly replenished; major source of drinking water and irrigation.

  • Surface water (lakes, rivers, streams): Easily accessed by ecosystems and humans; linked to runoff and precipitation.

  • Atmosphere: Holds water vapor temporarily; forms clouds and drives weather.

  • Biosphere: Water in living organisms; small but part of evapotranspiration and plant uptake.

Key Processes

Evaporation

  • Liquid water → water vapor due to solar heat.

  • Primary way water enters the atmosphere.

  • Rate increases with higher temperature, wind, and low humidity.

Transpiration

  • Plants release water vapor through stomata in leaves.

  • Major source of atmospheric water in vegetated areas.

  • Combined with evaporation → evapotranspiration, the total water loss from land to atmosphere.

Sublimation

  • Ice or snow → water vapor without becoming liquid.

  • Occurs in cold, dry regions like glaciers and high mountains.

Condensation

  • Water vapor cools → liquid droplets or ice crystals form clouds or dew.

  • Releases latent heat, warming the atmosphere.

  • Precursor to precipitation.

Precipitation

  • Water falls to Earth as rain, snow, sleet, or hail.

  • Replenishes freshwater on land, rivers, lakes, and groundwater.

Infiltration

  • Water soaks into soil, replenishing soil moisture.

  • Rate depends on soil type, vegetation, and land use.

Percolation / Groundwater Recharge

  • Water moves deeper through soil and rock into aquifers.

  • Creates long-term underground freshwater storage.

  • Movement can take years to centuries.

Runoff / Surface Flow

  • Water flows over land into rivers, lakes, and oceans.

  • Occurs when precipitation exceeds infiltration capacity.

  • Carries nutrients and pollutants from land to water bodies.

Plant Uptake

  • Roots absorb water from soil or groundwater for photosynthesis.

  • Links water cycle to the biosphere and food webs.

Human Impacts

  • Deforestation: Less transpiration and interception → more runoff and erosion.

  • Urbanization: Impermeable surfaces → reduced infiltration, increased flooding.

  • Groundwater extraction: Over-pumping lowers water tables; depletes aquifers.

  • Agriculture: Irrigation alters infiltration/runoff; fertilizers and pesticides contaminate water.

  • Dams / Water Diversions: Change flow patterns; increase surface evaporation; affect ecosystems.

  • Climate change: Alters precipitation patterns, increases evaporation, accelerates glacier melting.

Exam Tip: Know cause-and-effect chains. Example: Deforestation → reduced infiltration → increased runoff → floods.

Quick Summary of Flow

Evaporation + Transpiration → Condensation → Precipitation → Runoff / Infiltration → Groundwater / Surface Water → Plant Uptake → Evapotranspiration → Repeat