5.2

• A drainage basin is the area drained by a river and its tributaries. Surrounding the drainage basin is a ridge of higher land that marks the boundary of the basin called the watershed. A drainage basin is a subsystem within the global hydrological cycle.

  A drainage basin is an open system as it has inputs, stores, flows and outputs with inputs from and outputs to other systems.
• Drainage Basin Inputs


  Precipitation - relief, frontal, convectional. Highest precipitation are found in the tropics due to the ITCZ
• Tributary basins flows – major rivers e.g the Amazon, have tributary basins such as the Negro, which in turn are made up of even smaller tributary basins. These add water to the drainage basin system.
• Drainage Basin Flows

  Interception – vegetation prevents water. Leaves, branches and stems catch the water and this can be evaporated back into the atmosphere. Some of the intercepted water will flow down stems, branches and trunks (stem flow) or will drip off the vegetation (through fall) and eventually reach the ground. The rate of interception is influenced by two factors:

  • Precipitation – light + short precipitation = interception will be greater
  • Vegetation type and cover – coniferous trees have denser foliage, deciduous trees have broader leaves, but lose their leaves in cold climates.
• Infiltration – movement of water through pores in the soil. Rate of it will be affected by:

  • soil type – sand based soils have larger pores, clay soils are far less porous.
  • Precipitation – if precipitation rate is greater than infiltration capacity then water will not infiltrate
  • Vegetation – roots help break up the soil and also pull water towards them
  • Slope gradient – steeper the slope, faster the infiltration capacity.
  • Antecedent moisture – amount of water already in the soil

  Throughflow is the movement of water in a soil; it can be vertical or lateral and is aided by gravity.
• Drainage Basin Flows

  Direct runoff (overland flow) – water that flows over the surface of the ground. There are two types:

  • Saturated overland flow – if soil is fully saturated then further precipitation won't infiltrate the soil and will stay on surface forming puddles or, if on a slope it will flow.
  • Infiltration-excess overland flow – If rainfall intensity exceeds infiltration capacity, water will remain on the surface forming puddles or if on a slope it will flow.
• Drainage Basin Flow

  Percolation and Groundwater – movement of water into permeable rock. Water will fill the spaces within the rock, creating groundwater storage and an aquifer. This happens if there's an impermeable layer of rock below. The upper layer of the saturated zone is called the water table. Water may move laterally to form groundwater flow. Rate of percolation + groundwater flow depends on the rock structure. Porous rocks - sandstone, limestone. Granite is impermeable. If the stratum is angled then gravity may cause the groundwater to move more quickly.
• Drainage Basin Output

  Evapotranspiration – the total amount of moisture removed from a drainage basin by evaporation and transpiration. Evaporation - water is converted into a gas

  Transpiration - the way plants draw up water from the soil and then lose this water through stomata in the leaves. The rate of evapotranspiration is influenced by:

  Temperature – amount of solar radiation

  Wind – reduces the level of humidity in the air

  Vegetation cover – both type and season will influence rates

  Soil moisture content – makes more water available for evaporation and transpiration.
• Drainage Basin Output

  Channel flow – the water that flows in a stream or river. The discharge is measured in cumecs and is dependent on the amount of precipitation falling directly into the channel, and on the contribution from drainage basin flows via surface runoff, throughflow and groundwater flow.
• Human activity can disrupt the drainage basin system

  Deforestation – removal of trees will reduce interception and evapotranspiration which leads to increased surface runoff, leaching of soils, soil erosion, flooding, silting of rivers, drought.

  Cloud seeding is the attempt to change the amount or type of precipitation by dispersing substances (e.g silver iodide) into the air. These act as hygroscopic nuclei leading to faster condensation and therefore greater potential for rainfall.
• Human activity can disrupt the drainage basin system

  Urbanisation – impermeable surfaces reduce infiltration and increase surface runoff + throughflow through artificial drains, this increases river discharge leading to a greater risk of flooding.

  Dam – they increase surface water stores and evaporation. They also reduce downstream river discharge. e.g The Aswan Dam

  Groundwater abstraction – sometimes groundwater is abstracted faster than it is replaced which reduces groundwater flow and lowers the water table.