CGP Drainage Basins

• Drainage Basins


  Natural Systems
• Drainage basins

  Open, local hydrological cycles
• Drainage basin

  1. Rain falling on the land flows into the river

  2. Boundary is the watershed - any precipitation falling beyond the watershed enters a different drainage basin
• Drainage basins


  • Open systems with inputs and outputs
• Inputs to drainage basin

  1. Precipitation

  2. Outputs from drainage basin: Evaporation, transpiration, river discharge
• Inputs, Stores, Flows and Outputs in Drainage Basins

  • Inputs: Precipitation
  • Storage: Interception, vegetation, surface, soil, groundwater, channel
  • Flows: Infiltration, overland flow, throughfall, stemflow, throughflow, percolation, groundwater flow, baseflow, interflow, channel flow
  • Outputs: Evaporation, transpiration, evapotranspiration, river discharge
• Interception


  Precipitation landing on vegetation or structures before reaching soil
• Vegetation storage

  Water taken up by plants
• Surface storage

  Water in puddles, ponds, lakes
• Soil storage


  Moisture in soil
• Groundwater storage

  Water stored in soil or rocks, water table is top of zone of saturation
• Channel storage


  Water held in river or stream channel
• Infiltration


  Water soaking into soil
• Overland flow (runoff)


  Water flowing over land surface
• Throughfall


  Water dripping from one leaf or plant part to another
• Stemflow

  Water running down plant stem or tree trunk
• Throughflow


  Slow downhill movement of water through soil
• Percolation


  Water seeping down through soil into water table
• Groundwater flow


  Slow movement of water below water table through permeable rock
• Baseflow


  Groundwater flow that feeds into rivers
• Interflow


  Water flowing downhill through permeable rock above water table
• Channel flow


  Water flowing in river or stream (river discharge)
• Evaporation


  Water turning into water vapour
• Transpiration

  Evaporation from within leaves
• Evapotranspiration


  Evaporation and transpiration together
• Potential evapotranspiration (PET)


  Amount of water that could be lost by evapotranspiration
• Actual evapotranspiration


  What actually happens, e.g. in a desert PET is high but actual transpiration is low
• Water balance

  1. Inputs (precipitation) and outputs (channel discharge, evapotranspiration) balanced

  2. Affects amount of water stored in basin

  3. In wet seasons, precipitation exceeds evapotranspiration creating a surplus

  4. In drier seasons, precipitation is lower than evapotranspiration depleting ground stores