Hydrology and the Water Cycle

Hydrological Cycle

• Water continuously cycles, a closed system.
• Evaporation (liquid to gas) from bodies of water (e.g., seas, lakes) adds moisture to the atmosphere.
• Rising water vapor cools, condenses (gas to liquid), and forms clouds.
• Precipitation (e.g., rain, snow) returns water to the surface,
• Water may flow across land as surface runoff or channel flow in streams/rivers.
• Some water is intercepted by vegetation before reaching the ground.
• Water eventually returns to the sea/lakes, completing the cycle.

Drainage Basin

• A drainage basin is an open system with inputs, storage, transfers, and outputs.
• Inputs: Precipitation (rain, snow).
• Storage: Lakes, rivers, vegetation interception.
• Transfers: Surface runoff, channel flow, infiltration, throughflow.
• Outputs: Evaporation, surface runoff into seas/oceans.

Human Impact on the Hydrological Cycle

• Mining reduces storage capacity of lakes/rivers by causing silting.
• Reduced vegetation increases surface runoff.
• Irrigation reduces river flow and lowers water tables, increasing evaporation.
• Deforestation increases surface runoff and extreme river flows.
• Urbanization increases impermeable surfaces, accelerating surface runoff and leading to higher river levels.

Urban vs. Rural Hydrographs

• Urban Area:
  • Steeper rising limb due to less interception and faster runoff.
  • Higher peak discharge due to impermeable surfaces.
  • Short lag time due to accelerated runoff.
  • Steeper recessional limb due to quicker runoff.
• Rural Area:
  • Gentler rising limb due to vegetation interception and slower runoff.
  • Lower peak discharge due to absorption and slower movement.

River Valency Storm Hydrograph (16th August)

• Discharge Changes:

  • Baseflow around 2 cumecs.
  • Slow rise in discharge initially (07:00-09:00).
  • 5-hour lag time.
  • Steep rising limb, peak discharge 100 cumecs at 18:00.
  • Rapid falling limb to baseflow between 18:00-22:00.

• Reasons for Changes:

  • Initial slow rise due to early rainfall and vegetation interception
  • Soil storage exceeded leads to steeper rise; impermeable surfaces cause rapid runoff.
  • Steep catchment leads to rapid overland flow.
  • Falling limb results from lack of further rainfall.

Waterfall Formation

• Hard rock overlies soft rock.
• Hydraulic action & abrasion erode soft rock.
• Plunge pool forms.
• Continued erosion undercuts overhang of hard rock which collapses.
• Waterfall retreats upstream over time.

Meander Formation

• Winding curves/bends in middle/lower course rivers.
• Hydraulic action/abrasion erode outer bends.
• Faster flow on outer bend, slower on inner bend.
• Deposition on inner bend & erosion on outer bend.
• Meanders migrate downstream.

Oxbow Lake Formation

• Erosion (hydraulic action/abrasion) continues on outer meander bends and deposition on inner bends
• Flood event creates a new, straighter channel.
• Original meander is cut off forming an oxbow lake.

V-Shaped Valley Formation

• Rivers in mountainous areas erode vertically quickly due to high flow velocity.
• Hydraulic action and abrasion erode river channel.
• Freeze-thaw weathering impacts valley sides.
• Eroded material transported by river deepens channel, widening valley in V-shape.