River Discharge

River Discharge

1. Hydrographs

   • Peak discharge
     • Highest point on the graph when rivers discharge is at its greatest
   • Lag time
     • Delay time between peak rainfall and peak discharge
     • Delay occurs because it takes time for rainwater to flow into the river
   • Rising limb
     • Part on the graph up to the peak discharge
     • River discharge increases as water flows into the river
   • Falling limb
     • Part of the graph after peak discharge
     • Discharge is decreasing because less water is flowing into the river

2. River Discharge is the volume of water a river can carry

   • Measured in cubic metres per second (cumecs)

3. Storm hydrograph affected by physical factors

   • Drainage basin characteristics
     • Larger drainage basins
       • Catch more precipitation
         • So higher peak discharge
       • Smaller drainage basins though, generally have shorter lag time
     • Steep sided
       • Shorter lag times as water flows more quickly
     • Circular Basins
       • All points on the watershed are roughly the same distance from the point of discharge
         • Means lots of water will reach the measuring point at the same point at the same time, increasing peak discharge
       • More likely to have flashy flood
   • The amount of water already present in the drainage basin
     • If the grounds already waterlogged (soil can't absorb any more) then infiltration is reduced and surface runoff increases.
   • Rock type
     • Impermeable rocks don't store water or flow through them
       • Reduces infiltration and increases surface runoff, reducing lag time
   • Soil type
     • Sandy soils allow more infiltration, but clay soils have low infiltration rates
       • Low infiltration rates increases surface runoff off, reducing lag time, increasing peak discharge
   • Vegetation
     • Vegetation intercepts precipitation
       • Increasing lag time
   • Precipitation
     • Intense storms generate more precipitation and so greater peak discharges than light showers
   • Temperature
     • Hot dry condition and cold freezing conditions results in hard ground
       • Increases surface runoff, reducing lag time, increasing peak discharge

4. Human activity affects the hydrograph

   • In urban areas much of the soil is covered in man made impermeable materials like concrete.
     • Water can't infiltrate, which increases surface runoff, shorter lag time, increased peak discharge
   • Man made drainage systems
     • Water flows down drains before it can evaporate or infiltrate.
       • Causing shorter lag time and increased peak discharge