2.1.2 - The Drainage Basin as a System

2.1.2 - The Drainage Basin as a System

1. Open system

   • Water can enter and leave the system
   • The drainage basin is a subset of the global hydrological cycle​ It is defined as a catchment area forming part of the Earth's surface area which is drained by a particular stream or river ​ It is an open system which allows energy and matter to be transferred across its boundary (the watershed or drainage divide) from external areas

2. Input

   • Water entering the system
     • Precipitation - any product of the condensation of atmospheric water vapor that falls under gravity.

3. Flow

   • The movement of water between components
     • Throughfall: Rainwater dripping from leaves and branches towards the ground (rain drops fall through the plants)
     • Stemflow: Water flows along tree trunks and plant stems towards the ground
     • Infiltration: Downward movement of water from ground surface into the soil
       • Throughflow: Sideways movement of water through the soil pores
       • Percolation: Downward movement of water from soil to rock layer
         • Groundwater flow: Movement of water through the rock layer
     • Overland flow: Sheet of water moving along the surface
       • Two forms of overland flow
         • Infiltration-excess: Intense rainfall means not all water can infiltrate soil, therefore runs along the surface
         • Saturation-excess: Excessive rainfall saturates soil so no more water can infiltrate
     • Channel flow: movement of water through the river channel

4. Store

   • Where water is held in the system
     • Interception: Water held on leaves and plant surfaces
     • Vegetation store: Water within the biomass (plants and animals)
     • Surface store: Water on the ground e.g. puddles
     • Soil moisture store: Water held in soil
     • Channel store: Water held in river channels
     • Groundwater store: Water stored within rock layer

5. Outputs

   • Water leaving the system
     • Channel discharge: Water leaving via river channels
     • Evapotranspiration
       • Evaporation
         • Change of state from water to gas (caused by temperature and wind speed)
       • Transpiration
         • Water vapour released from plants

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