Hydrologic Cycle (Water Cycle) Lecture Notes

Overview of the Water Cycle

• Continuous circulation of water within the Earth–atmosphere system.

  • Total global water volume stays essentially constant.

  • Spatial & temporal distribution among processes is highly dynamic.

• Core processes (in order of typical sequence):

  • Evaporation (incl. sublimation)

  • Transpiration → (collectively with evaporation) Evapotranspiration

  • Condensation

  • Precipitation

  • Runoff (surface & subsurface)

• System integrates land surface, oceans, cryosphere, biosphere, and atmosphere.

Evaporation & Sublimation

• Definition: transfer of water from liquid (or solid) to vapor at the surface.

• Molecular basis: high-energy (high-kinetic) molecules escape the cohesive forces at the surface.

• Primary source: oceans; additional sources include soils, snow, and ice.

• Sublimation: direct phase change from solid (snow/ice) → vapor.

• Controlling variables:

  • Temperature (↑T ⇒ ↑evaporation rate)

  • Relative humidity (↓RH ⇒ ↑evaporation potential)

  • Wind speed (↑wind ⇒ removes saturated boundary layer)

  • Solar radiation (provides latent-heat energy)

• Measurement challenges: true evaporation rates can be obtained only at point locations (e.g., evaporation pans, eddy-covariance towers).

Transpiration & Evapotranspiration (ET)

• Transpiration: evaporation through stomata in plant leaves.

• For hydrologic accounting, combine all surface/vegetative losses as evapotranspiration (ET).

• ET provides the major atmospheric moisture supply over vegetated land areas.

• Practical implications:

  • Crop-water requirements

  • Climate feedbacks via latent heat flux

Atmospheric Storage & Condensation

• Water vapor is the primary form of atmospheric moisture.

  • Almost entirely confined to the troposphere (lowest 6\text{–}8 miles \,(10\text{–}13\;\text{km})).

• Condensation: transition \text{vapor} \rightarrow \text{liquid} once air exceeds saturation capacity.

  • Triggers: cooling (adiabatic lifting, radiative loss) or mixing of air masses with different temperatures.

• Products of condensation:

  • Dew, frost, fog (e.g., King Range fog along California coast)

  • Clouds → eventual precipitation

  • Requires condensation nuclei (dust, salt, pollutants).

Precipitation

• Water released from condensed vapor returns to surface as rain, snow, sleet, hail.

• Indonesia example: heavy rainfall year-round, demonstrating climate–cycle linkage in equatorial regions.

Fate of Precipitation at the Surface

• Four principal pathways once precipitation reaches land:

  1. Evaporation/ET back to atmosphere.

  2. Interception by vegetation canopy → subsequent evaporation.

  3. Infiltration into soil → percolation to groundwater.

  4. Surface runoff → rivers → ocean.

• Groundwater contribution:

  • Infiltrated water may re-emerge as baseflow to streams.

  • Flow rates slow & variable: a few mm to a few meters per day.

  • Investigated via tracer studies & remote sensing.

• Hydrologic budgeting equation (conceptual):
  P = ET + Q + \Delta S
  where P = precipitation, ET = evapotranspiration, Q = runoff, \Delta S = change in storage (soil, groundwater, snow/ice).

Runoff & Measurement Techniques

• Runoff quantified with stream gauges; results plotted on hydrographs (discharge vs. time).

• Integrates both surface flow and delayed groundwater inputs.

Cryosphere: Ice & Snow in the Water Cycle

• Forms: frost, sea ice, glacier ice, permafrost.

• Soil-moisture freezing in tundra forms permafrost layers.

• Historical extent:

  • \sim18,000 years ago (Last Glacial Maximum): glaciers/ice caps covered \approx \tfrac{1}{3} of land surface.

  • Present day: \approx12 \% of land surface remains ice-covered.

• Example feature: Perito Moreno Glacier, Patagonia (Argentina) – showcases active ice dynamics.

• Importance:

  • Stores large freshwater volumes.

  • Influences sea level, albedo, and regional climate.

Illustrative Examples & Case Studies

• Fog along King Range (California): visual evidence of condensation near coastal upwelling zones.

• Indonesia’s persistent rainfall: highlights interaction between warm sea-surface temperatures, convection, and hydrologic throughput.

• Patagonia glaciers: demonstrate cryospheric storage and potential melt-water contributions.

Ethical, Practical & Real-World Implications

• Water-resource management: understanding ET and runoff critical for agriculture, urban supply, flood control.

• Climate change impacts:

  • Altered evaporation/precipitation regimes.

  • Retreat of glaciers (e.g., Perito Moreno) affects downstream water availability.

• Groundwater depletion vs. recharge balance – necessitates sustainable extraction policies.

• Fog-harvesting, cloud seeding, and other geoengineering practices rely on condensation principles.

Connections to Foundational Principles & Previous Material

• Thermodynamics: phase changes governed by latent heat, saturation vapor pressure.

• Fluid dynamics: wind shear and turbulence enhance evaporation and mixing.

• Geology & soil science: infiltration capacity depends on texture, structure, and antecedent moisture.

• Ecology: transpiration links plant physiology with atmospheric humidity.

Numerical & Statistical References

• Atmospheric water vapor mostly below 10\text{–}13\,\text{km} altitude.

• Groundwater velocities: \approx 10^{-3}\text{–}10^{0}\,\text{m day}^{-1}.

• Present ice cover: 12\,\% of terrestrial surface compared to \sim33\,\% during the Last Glacial Maximum (\approx18{,}000 years BP).

• Typical fog droplet radius: 10^{-6}\text{–}10^{-5}\,\text{m} (context for condensation nuclei effectiveness).

Key Terms & Definitions (Quick Reference)

• Evaporation: \text{liquid} \rightarrow \text{vapor} phase change at free surface.

• Sublimation: \text{solid} \rightarrow \text{vapor} without intermediate liquid phase.

• Transpiration: vapor flux through plant stomata.

• Condensation: \text{vapor} \rightarrow \text{liquid} when air exceeds saturation.

• Precipitation: gravity-driven fall of condensed water/ice particles to surface.

• Runoff: lateral water movement over/through land to streams & oceans.

• Infiltration: downward entry of water into soil.

• Permafrost: perennially frozen ground (>2 consecutive years).

Citation: Encyclopaedia Britannica, “Water Cycle,” last revised 19 April 2025; accessed 29 May 2025.