Drainage Basin

• Drainage Basin


  Area of land drained by a river and its tributaries. Its boundary is marked by a ridge of high land beyond which any precipitation will drain into adjacent basins.
• The drainage basin is a convenient unit for analysis as it is normally well-defined topographically
• The drainage basin can be studied as a series of nested units of increasing size
• The drainage basin has a system for which inputs and outputs of mass and energy can be defined and measured
• Change that occurs in any portion of a drainage basin can affect the entire system
• Parts of a drainage basin

  • Source of river
  • Catchment area
  • Watershed
  • Tributary
  • Confluence
  • Main river channel
  • Mouth
• At a global hydrological cycle scale, the drainage basin is a closed system
• At a regional/local scale, the drainage basin is an open system
• Water balance


  Shows the annual balances between inputs and outputs at a particular time
• Water balance formula


  P = Q + E ± S (where P = precipitation, Q = runoff, E = evapotranspiration, S = changes in water storage)
• The water balance is influenced by a range of natural & human factors
• The water balance is used to monitor the availability of H2O held in stores
• Potential Evapotranspiration (PET)


  The amount of evaporation that would occur if a sufficient water source were available
• Water Balance Graph


  1. A: PPT > PET (input > output), surplus of water

  2. B: PET > PPT (output > input), deficit in water

  3. C: PET max, all soil moisture used up

  4. D: PET declines but continues to be > PPT, deficiency in soil moisture

  5. E: PPT > PET, excess water used to recharge soil moisture

  6. F: PPT > PET, soil moisture storage restored
• Inputs of water


  • Precipitation (rainwater, meltwater)
  • Flows from tributary basins
• Outputs of water


  • Evapotranspiration
  • River discharge
• Precipitation


  Source of water to a drainage basin, expressed in mm
• Precipitation intensity


  Rate of rainfall measured in depth over time (usually mm/hr), greater intensity = shorter duration → may lead to severe flooding
• Evapotranspiration


  Combination of evaporation & transpiration from earth's land surface to the atmosphere, most important aspect of water loss
• River discharge


  Movement of water in channels such as streams and rivers, refers to any flow confined by topography to flow downhill in a trough-like depression
• Variations in the balance between input & output in the tropics

  1. Af climate: net surplus water balance, PPT constantly > PET

  2. Am climate: PET > PPT Jan-May, PPT > PET May-Oct, net surplus water balance

  3. BWh climate: low PPT throughout, net deficit water balance, PET is high but PPT added to soil only penetrates a few cm before being absorbed or evaporated
• Water storages


  • Channel storage
  • Groundwater storage
  • Interception storage
  • Soil water storage
• Groundwater storage


  Water located beneath the ground surface in soil pore spaces within bedrock and regolith, as the amount of groundwater increases/decreases, the water table rises/falls accordingly
• Interception storage


  Water caught by vegetation and stored on the canopy, affected by type & density of vegetation hence seasonal changes
• Soil water storage


  The amount of water held in the soil at any particular time, either by gravity or capillary action, depends on soil properties
• Pathways


  • Baseflow (groundwater flow)
  • Overland flow (infiltration excess flow, saturation overland flow)
  • Infiltration
• Baseflow


  Lateral/horizontal movement of groundwater in the zone of saturation, slowest speed, 50% of water that percolates down to shallow ground water contributes to baseflow
• Overland flow


  Rainwater/meltwater that travel over the land surface from an area of higher elevation to an area of lower elevation towards the stream channels
• Infiltration excess overland flow (IEF)


  Occurs when rainfall intensity is very high and soil has no time to absorb water, common in arid/semi-arid areas and urban areas with impervious surfaces
• Saturation overland flow (SOF)

  Occurs when soil is already saturated and rain continues to fall, unable to seep into the ground, found at the base of slopes
• Infiltration


  Downward entry of water into the surface of a soil, drawn in by gravity and capillary action, rate of entry limited by diameter of pores and cohesiveness between particles
• Infiltration rate


  Measures the amount of water that passes through a given unit of soil
• Infiltration capacity


  Maximum rate at which water can be absorbed at a steady state, may range from 5mm/h (clays) to 50mm/h (sands)
• Infiltration is critical as it regulates water, determines soil moisture storage, supplies water for other sub-surface flows and storages, and delays arrival of water into the river channel
• Top of slope


  Steep areas
• Bottom of slope


  Gentle areas
• Infiltration


  1. Downward entry of water into the surface of a soil

  2. Drawn into the soil by the forces of gravity and capillary action
• Capillary action


  Thin film of water enveloping the soil particles via surface tension, almost like suction → can pull upwards or downwards
• Porosity


  The volume available for additional water depends on the porosity of the soil
• Infiltration rate


  Rate at which water enters the soil