Overview

The water cycle (hydrological cycle) describes the continuous movement of water on, above, and below the surface of Earth. It is driven by solar energy and gravity, cycling water through various reservoirs: atmosphere, oceans, rivers, lakes, soil, and living organisms.

Key Processes

1. Evaporation

  • Water changes from liquid to vapor, mainly from oceans, lakes, and rivers.
  • Driven by solar radiation.
  • Plants also contribute via transpiration.

2. Condensation

  • Water vapor cools and forms liquid droplets, creating clouds.
  • Occurs when air rises and cools to its dew point.

3. Precipitation

  • Water droplets in clouds grow and fall as rain, snow, sleet, or hail.
  • Returns water to Earth’s surface.

4. Infiltration

  • Water soaks into the soil, replenishing groundwater.
  • Influenced by soil type, vegetation, and land slope.

5. Runoff

  • Water flows over land into rivers, lakes, and oceans.
  • Occurs when precipitation exceeds infiltration.

6. Sublimation & Deposition

  • Sublimation: Ice/snow changes directly to vapor (e.g., glaciers).
  • Deposition: Vapor turns directly into ice without becoming liquid.

7. Transpiration

  • Plants release water vapor through leaves.
  • Major contributor to atmospheric moisture.

Diagram

Water Cycle Diagram

Key Equations

  1. Evaporation Rate (E):

    E = K × (es - ea)
    • E: Evaporation rate
    • K: Empirical constant (depends on wind, temperature)
    • es: Saturation vapor pressure
    • ea: Actual vapor pressure

  2. Water Balance Equation:

    P = Q + E + ΔS
    • P: Precipitation
    • Q: Runoff
    • E: Evapotranspiration
    • ΔS: Change in storage (soil, groundwater)

Surprising Facts

  1. Atmospheric Rivers:
    Narrow bands of concentrated moisture in the atmosphere can transport more water than the Mississippi River—sometimes causing extreme precipitation events.

  2. Bacterial Survival:
    Some bacteria can survive in extreme environments (deep-sea vents, radioactive waste), influencing water chemistry and cycling even in hostile locations.

  3. Hidden Groundwater:
    Over 30% of the world’s freshwater is stored underground, unseen but crucial for ecosystems and human consumption.

Global Impact

Climate Regulation

  • Water cycle moderates global temperatures via heat transfer during evaporation and condensation.
  • Influences weather patterns and precipitation distribution.

Ecosystem Health

  • Supports plant growth, nutrient cycling, and habitat stability.
  • Disruption (e.g., drought, floods) affects biodiversity and food security.

Human Activities

  • Agriculture, industry, and urbanization alter natural water flows.
  • Over-extraction of groundwater leads to land subsidence and reduced water quality.

Pollution & Contamination

  • Pollutants enter the cycle via runoff, precipitation, and infiltration.
  • Waterborne contaminants can travel globally, impacting health and ecosystems.

Climate Change Effects

  • Intensifies the water cycle: more evaporation, heavier rainfall, frequent droughts.
  • Alters snowmelt timing, glacier retreat, and sea level rise.

Recent Research

A 2022 study in Nature Reviews Earth & Environment (Zhou et al., 2022) found that the global water cycle is accelerating due to warming temperatures, with increased evaporation and precipitation rates, leading to more extreme weather events and shifting water availability.

Most Surprising Aspect

Microbial Influence:
Microorganisms, including extremophile bacteria, play a critical role in the water cycle. They drive chemical transformations in groundwater, influence cloud formation by acting as cloud condensation nuclei, and even survive in environments previously thought to be inhospitable (e.g., deep-sea vents, radioactive waste). This microbial activity affects water quality, nutrient cycling, and even precipitation patterns.

Summary Table

Process Description Key Impact
Evaporation Liquid → Vapor Moisture to atmosphere
Condensation Vapor → Liquid Cloud formation
Precipitation Water falls to Earth Replenishes surface water
Infiltration Water enters soil Groundwater recharge
Runoff Surface flow River/lake input
Transpiration Plant vapor release Atmospheric moisture
Sublimation Ice → Vapor Glacier mass loss

Revision Checklist

  • Understand each process and its drivers.
  • Learn key equations and their variables.
  • Know global impacts and recent research findings.
  • Recognize the role of microorganisms in the cycle.
  • Be aware of human influence and climate change effects.

Citation

  • Zhou, T., et al. (2022). “Acceleration of the global water cycle under climate change.” Nature Reviews Earth & Environment, 3, 256–273. Link

End of Revision Sheet