APES Notes 1.7 - Hydrologic (Water) Cycle

Main Ideas / Concepts Covered (Hydrologic / Water Cycle: APES Notes 1.7)

Purpose of the Lesson

• Be able to explain the steps and reservoir interactions in the hydrologic (water) cycle.
• Practice the scientific skill of explaining differences/relationships between environmental concepts using a visual model.

Core Definition

• The hydrologic cycle is the movement of water in different states (solid, liquid, gas) between sources (reservoirs).

How to Describe Any Water-Cycle Step

When analyzing a specific step, identify:

• State of matter of the water (solid / liquid / gas)
• Source/reservoir the water is leaving
• Destination/reservoir the water is entering

Example Processes and Reservoir Interactions

Precipitation

• Water moves from atmosphere (gas) → Earth’s surface
• Condensation forms clouds, and then water falls as liquid rain
• After reaching the surface, water may:
  • Become surface runoff (to bodies of water), or
  • Infiltrate into the ground → become groundwater (aquifers)

Energy Driver

• The hydrologic cycle is driven by solar energy
• Sun heating causes water to warm and eventually evaporate

Major Water Reservoirs (Highlighted)

• Ocean = largest reservoir of Earth’s water
• Freshwater reservoirs (smaller but critical):
  • Polar ice caps / glaciers
  • Mountain ice caps
  • Groundwater / aquifers
• Humans generally rely on freshwater (not ocean saltwater) for:
  • Agriculture
  • drinking

Evaporation-Related Processes

Evaporation (general concept / “vaporization”)

• Liquid water → gas/vapor into the atmosphere
• A key way water leaves Earth’s surface

Transpiration (plants)

• Plants draw water from the ground via roots and move it through stems to leaves
• Leaves release water vapor through stomata (pores)

Mechanism described:

• Stomata open → sun energy warms leaf → some leaf water evaporates
• Evaporation creates lower water pressure in leaves than in roots
• This pressure difference pulls water upward from roots to leaves

Evapotranspiration (combined term)

• The total water leaving an ecosystem through:
  • Transpiration + evaporation
• Both are described as sun-driven:
  • Sun heats soil/water → evaporation
  • Sun warms leaves and stomata open → transpiration

Runoff vs. Infiltration (Precipitation Outcomes)

When precipitation reaches Earth:

• Runoff (surface flow)
  • Water flows across the surface → into surface water (lakes, rivers, ponds)
• Infiltration
  • Water soaks into the ground → moves toward aquifers (underground water stores)

Why it matters for humans/ecosystems:

• Runoff can recharge surface waters used for:
  • drinking
  • agriculture
• Runoff and infiltration also support freshwater availability via recharge
  • Groundwater is especially important in dry areas where humans use wells

Permeability Concept

• Groundwater recharge depends on permeability (how easily water soaks through ground cover)
• More permeable surface (e.g., grass/soil):
  • More infiltration → more aquifer recharge
• Less permeable surface:
  • Less infiltration → more becomes runoff

Tradeoff Noted

• Runoff can carry and collect pollutants as it flows into surface water bodies.

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Methodology / Instructions Presented (FRQ Practice)

Task for the Viewer (Scientific Skill: Visual Model Explanation)

• Look at the water-cycle diagram with four labeled letters: A, B, C, D
• Choose one letter
• For the chosen letter:
  • Identify the process shown
  • Describe how water moves between reservoirs by that process (which reservoir it leaves and which it enters)

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Speakers / Sources Featured

• Mr. Smeeds