Hydrological Cycle - The Local (Drainage Basin) Hydrological Cycle

  • Rain: Frozen precipitation melts and reaches the ground as rain.
  • Freezing Rain: Frozen precipitation melts in warm air. Rain falls and freezes on cold surfaces
  • Sleet: Frozen precipitation melts in the shallow warm air then, refreezes into sleet before reaching the surface
  • Snow: Snow falls through cold air and reaches the surface

Types Of Rainfall:

Relief Rainfall – Mountains:

  1. Warm wet air is forced to rise over the high land
  2. As the air rises, it cools and condenses. Clouds will form and precipitation will occur.
  3. Drier air descends and warms
  4. Any moisture in the clouds evaporates.

Convectional Rainfall – Tropics, Equator, 5-10 Degrees North – South:

  1. Sun heats the grounds and warm air rises
  2. As the air rises, it cools and condenses to form clouds
  3. Large cumulonimbus clouds are formed
  4. Heavy rain storms occur with thunder and lightning.

Frontal Rainfall – Most Common In The UK:

  1. Warm air meets an area of cold air
  2. Warm air is forced over the cold air
  3. The air meets and is condensed and water vapour cools
  4. Clouds form and precipitation occurs.

Physical Factors In A Drainage Basin:

  • Drainage Basin: An area that collects all the precipitation so that it will flow out of the same river channel.
    • Climate is one of the most important factors as it inputs to the drainage basin. Desert climates have low inputs, tropical rainforest climates e.g monsoon season in India.
    • No soil leads to a faster surface runout by direct overland flow. The deeper the soil the more infiltration and through flow which slows runoff rate. Saturated soil leads to a water table being the surface of the land leading to overland flow.
    • Geology affects the flow by determining how much runoff or percolation occurs to the groundwater. If a rock is very impermeable like granite than more runoff will occur. Soft rock leads to more percolation. Cracks and joints leads to little overland flow as it goes into the ground.
    • Relief is an important output. Steep drainage basins drain faster than flatter basins which means flow will occur faster. Steep basins mainly output water by flow. Flatter basins like swamps may have more output such as evaporation.

Human Disruption To The Water Cycle:

Deforestation:

  • Cutting down trees on a large scale
  • Reduces interception, vegetation storage and transpiration
  • Runoff increases leading to more soil erosion although infiltration may increase or decrease
  • Rainfall in places such as the Amazon rainforest decreases
  • Often forests are cut down to be turned into agricultural land such as grazing cattle, this is known as lane use change
  • Some forests are cut down as urban areas expand, changing land use from forests to towns and cities.
  • Urban areas are concrete, tarmac etc increase run off
  • Both of these changes can increase flooding, reduce infiltration like urban surfaces like roads and buildings are often impermeable.

Reservoirs:

  • Are man-made lakes where rivers are blocked with dams to create a large storage of fresh water
  • Provides drinking water and hydro-electric power.
  • They disrupt and change the water cycle in many ways e.g river channel becomes a store, residence times increase and evaporation increases
  • Helps to control flooding and droughts. The Three Gorges Dam is the largest dam in the world whilst the Yellow River in China is the largest for HEP.

Advantages & Disadvantages Of The Three Gorges Dam:
Advantages:
Disadvantages:
Flood Control: It protects Shanghai, Wuhan and Nanjing.
Water Pollution: A lot of the land around the area is eroding which leads to sand in the water.
Power Generation: On average 30% of the power in the local area comes from the Dam.
Migration Problem: Officials estimate that the reservoir will inundate two cities. 1.1 million people will have to resettle.
Cruise Ship Navigation: Makes ships going north a lot safer by having the ship locks
Environmental Problem: Habitats were destroyed during the construction and vegetation was also lost.


Abstraction Of Water From Stores:

  • Water can be abstracted from rivers, lakes, reservoirs and groundwater
  • If unsustainable, this lower the ground water level (water table)
  • This can lead to rivers, lakes and aquifers being depleted or even dry
  • Abstraction from rivers directly can cause environmental damage to ecology and reduce wetlands value (as less water flows into the wetland)
  • Industry could be impacted, and pollution is less diluted
  • Over-abstraction of ground water can also reduce supplies and lead to shortages for drinking and the environment

at