5.3A Water budgets

• • Drainage basins are self-adjusting systems to create balance
  • The balance between inputs and outputs is known as the water balance 
  • The water balance helps to understand the behaviour of individual drainage basins 
• Water balance equation:

  • precipitation (P) = total runoff (streamflow) (O) + evapotranspiration (E) +/- changes in storage (S)
• • 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
• The water balance = how much water is stored in a system
• Soil water budget:

  • Shows the balance between inputs and outputs of a soil store over a year
  • The budget depends on soil depth, type, texture and permeability 

  A) Soil water utilisation