1.2 Rainfall-discharge relationships within drainage basins

• Hydrograph Components: Rising Limb


  Shows an increase in discharge on a hyrdograph.

  
• Hydrograph Components: Peak


  Shows the maximum amount of water held in a channel.

  
• Hydrograph Components: Falling Limb


  Indicates when discharge decreases and the river's water level falls.

  
• Hydrograph Components: Lag time

  The time taken for rainfall to reach the river. A shorter lag time means more chance of flooding and vice versa.

  
• Hydrograph Factors: Drainage Basin Shape


  This can affect affect the time it takes for the water to reach the river.
• Hydrograph Factors: Gradient

  The steeper the gradient in an area, the shorter it can take for the water to reach the river system, resulting in a shorter lag time.
• Hydrograph Factors: Ground Permeability

  If the ground is permeable, it takes longer for the water to reach the river and overland flow is reduced, resulting in a longer lag time.
• Hydrograph Factors: Vegetation

  An area with high vegetation density will limit the speed at which the water can travel, resulting in a longer lag time.
• Climate: Precipitation type and intensity.


  Heavy storms increase rain intensity can decrease the lag time. However, different precipitation types like snow can extend the lag time as it takes longer to melt and enter the river.
• Climate: Temperature

  High temperatures in the summer can increase evapotranspiration rates, therefore reducing the total river discharge.
• Climate: Evaporation, Transpiration and Evapotranspiration.

  Evaporation and Evapotranspiration can reduce the total discharge of the river.
• Climate: Antecedent Moisture

  After prolonged storms, the ground can become saturated very quickly due to the high water table meaning there will be more discharge in the river.
• Drainage Basin Characteristics: Size and Shape

  Circular drainage basins have a shorter lag time than long and thin basins because the water has less distance to travel to reach a river.
• Drainage Basin Characteristics: Drainage Density

  Basins with a high drainage density will have a shorter lag time as more of the precipitation will find a stream that runs into a river.
• Drainage Basin Characteristics: Porosity and Permeability

  Areas with permeable soil will allow more water to infiltrate the soil and could lead to less surface runoff and a longer lag time. On the other hand, low permeability soils and rocks will promote surface runoff meaning lag time will be shorter.
• Drainage Basin Characteristics: Slopes

  Steep basins have a shorter lag time than shallow basins as water travels both overland and through the soil quicker as it is being assisted by gravity.
• Drainage Basin Characteristics: Vegetation Type

  Drainage basins with high vegetation density will have more interception, therefore a longer lag time as water is held back from entering the river.
• Drainage Basin Characteristics: Land Use

  Urban surfaces such as tarmac speed up runoff and reduce lag times, as does plowed fields.
• What affects drainage basins: Seasonal Variations (Spring and Summer)

  • More vegetation growth - higher interception rates and less soil saturation - infiltration rates are higher.
  • Precipitation is lower in many regions - less saturated soil and less water in channels.
  • If precipitation is too low, ground becomes hard and less permeable - encourages overland flow.
• What affects drainage basins: Seasonal Variations (Autumn and Winter)

  • Less vegetation growth and cooler temperatures - less interception and transpiration - higher rates of overland flow
  • Soil may be more saturated - more overland flow and higher river discharge.
  • Frozen ground may be impermeable and lead to overland flow - snow takes longer to melt so could lead to longer lag time.
• Land Use affecting Drainage Basins: Deforestation

  • Less interception by trees so surface runoff increases.
  • The soil is no longer held together by roots - water storage decreases.
• Land Use affecting Drainage Basins:

  • Livestock can tramble the ground reducing infiltration as pore spaces are lost.
  • Ploughing increases infiltration by creating a looser soil - decreases overland flow.
• Land Use affecting Drainage Basins:

  • Roads and buildings have impermeable surfaces, reducing infiltration and increasing overland flow.
  • Urban drainage reroutes large amounts of water to different areas - water outside natural drainage basin can flow into river - increases peak discharge.