Drainage Basin System (AQA A Level Geography)

• A drainage basin can be defined as:

An area of land drained by a river and its tributaries. A drainage basin can also be called a ‘catchment’

• A major part of the hydrological system, the open system of drainage basins drains all the water which lands on the Earth's surface
• Climate, vegetation, soil structure and land use influence the character and development of a drainage basin
• There are vast spatial and geological differences; which vary from microsystems of one river or stream to macro systems of many thousands of tributaries
• Drainage basins can cover many square miles/km over a number of countries e.g. the Nile River basin drainage area is more than 3 million km2 over 12 countries with 73 % of the drainage basin in Sudan and Egypt

Drainage basin features

• All drainage basins have some features in common:
  • Watershed
  • Source
  • Confluence
  • Tributary
  • Mouth

Drainage basin features

The drainage basin as an open system: note that the atmosphere can be both a store and a transfer – clouds store but also move water around the planet

• As an open system, drainage basins have inputs, outputs, stores and flows
• Inputs are the addition of water to a drainage basin through precipitation
  • Inputs vary throughout the year (rain, sleet, snow, location etc) and intensities (flood, drought, temperature etc) and  frequency (seasonal, monsoon etc
• Outputs are the losses of water from a drainage basin in various forms
  • River discharge is the volume of water passing a point in the river channel at a given unit of time and expressed as cubic metres per second or ‘cumecs’ (m³/sec)
• The interception by vegetation and buildings can prevent as much as 40% of precipitation from reaching the ground
  • It slows the passage of water to the surface and reduces the amount that reaches the ground – some or all will evaporate and the rest will take a longer route over leaves, trees etc. 
  • This is important, as it reduces the amount available for overland/surface flow and therefore, there is a reduction in soil erosion and flooding
• Surface storage occurs in natural features such as lakes, puddles, reservoirs and wetlands but also in urban areas because of impermeable surfaces 
  
  • Soil stores – water retained within the pore spaces of the soil called interstices
  • Groundwater stores water that has percolated and is held in rocks below the top of the water table
    • A slow method of water transfer that continues to supply water well after a precipitation event has happened
• Flows in a drainage basin can be either above or below ground
  • Above:
    • Throughfall is precipitation that makes it to the ground without interception by the plant canopy
    • Drip flow is water that flows off leaves and drips to the ground. Some leaves have developed drip tips and waxy surfaces
    • Trunk and stem: the flow of water down the stems of plants or trunks of trees
    • Overland surface flow is when water flows over the land surface. Two types: channel and sheet flow
      • Channel flow is where water flows in small channels or rills (<30cm width or depth) in a defined stream pattern 
      • Sheet flow is a layer of water on the surface, either due to excess surface flow from increased precipitation or water cannot infiltrate quickly enough
      • Or where water flows over impermeable surfaces—roads, clay soil, or impacted soil
      • Overland flow increases on slopes or when there is saturation of the underlying soil and rock

      • Hortonian overland flow describes the tendency of water to flow horizontally across land surfaces when rainfall has exceeded infiltration capacity and depression storage capacity. The water cannot enter the ground, so it runs straight off the surface.

  • Below:
    • Infiltration is where water enters small openings and pores in the ground from the surface. Infiltration capacity depends on factors such as soil type, antecedent conditions, etc.
    • Thin, frozen, compacted, or already saturated soils have a low infiltration rate
    • Infiltration will be reduced when the slope gradient increases as water flows over the surface quicker than it can infiltrate
    • Vegetation cover increases infiltration rates as they slow the rate of flow allowing infiltration to occur, and their roots break up the soil increasing potential pore spaces and channels
    • Throughflow is the lateral (sideways) movement of water through the upper soil, along lines of seepage called percaline
    • Percolation is where water flows down through the soil layers and underlying rock is pulled down through gravity. The rate of travel is determined through porosity (soil) and permeability (rock)
    • Baseflow / Groundwater flow: water that has infiltrated and percolated into the bedrock and below the water, table to feed springs, river channels and recharge aquifers (recharge will only occur when there is an excess of water)

Channel network

• Every drainage basin is covered by a network of tributaries which connect to the main river channel. The number of tributaries in a drainage basin is referred to as the drainage density:
  • Drainage basins with lots of tributaries have a high drainage density
  • Drainage basins with few tributaries have a low drainage density
  • Drainage density affects the magnitude of stores

Water balance 

• Drainage basins are self-adjusting systems to create balance
• If one part is 'out', then one or all the others will compensate
• The balance between inputs and outputs is known as the water balance 
• The water balance helps to understand the behaviour of individual drainage basins 
• The water balance can be shown using the formula:
  • precipitation (P) = total runoff (streamflow) (O) + evapotranspiration (E) +/- changes in storage (S)
    

• The water balance shows how much water is stored in a system
• The general water balance in the UK shows seasonal patterns
• In wet seasons, precipitation is greater than evapotranspiration which creates a water surplus
• Ground stores fill with water which results in increased surface runoff, higher discharge and higher river levels
• This means there is a positive water balance
• In drier seasons evapotranspiration exceeds precipitation, as plants absorb water, ground stores are depleted
• This produces a water deficit at the end of a dry season generating a negative water balance

Soil water budget

• The soil water balance is shown through a water budget graph
• It shows the balance between precipitation (input) and potential evapotranspiration (output) of the soil as a store, over a year
• The budget depends on soil depth, type, texture and permeability 
• The following is based on a typical UK soil budget

Graph showing changes in soil moisture stores, on a local scale, over a year - soil moisture deficit (SMD) is not shown here, however, rates of SMD in the UK's is increasing due to the changing climate