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Saturday, May 14, 2011


What is hydrograph?

A hydrograph is a graph that reflects the discharge of a river over a period of time. Discharge is the volume of water that flows about a certain point per unit of time.

Discharge: volume of water in the river at a given time.
Construction of storm hydrographs:
  • Rising limb: The rising flood water in the river
  • Peak flow: Maximum discharge in the river
  • Recession limb: Falling flood water in the river
  • Basing lag time: Time difference between the peak of the rain storm and the peak of the river
  • Base flow: Normal discharge of the river
  • Overland flow: Volume of water reaching the river from surface run off
  • Through flow: Volume of water reaching the river through soil and underlying rock layer
  • Storm flow = Overland flow + Through flow
Factors influencing Storm Hydrographs:

1. Size of the drainage basin (Area):
    Larger basin receives more pricipitation -- larger runoff
    Larger basin means water will take longer to reach the river -- longer lag time
2. Shape of the drainage basin:
    Elongated basin -- lower peak flow and longer lag time than circular one
3. Slop:
    Steep slop -- steeper rising limb and shorter lag time
4. Characteristics of the Drainage Basin:
    Permeable rocks and soil mean rapid infiltration and little overland flow -- longer lag time and shallow rising lime
5. Land use and Human impact:
     Afforestation -- intercepts the precipitation -- shallow rising limb and longer lag time
     Urbanization -- tarmac and concrete form impermeable surface -- steep rising limb and short lag time

River management: the presence of a dam will allow flow to be controlled, reducing flood risk and allowing rivers to gradually respond to heavy rainfall in a controlled way. 
A very good website for more information and practice:

Hope you have learnt something. Thank you :)
Xu Ao, JH405, 14

1. http://serc.carleton.edu/introgeo/socratic/examples/hydrosphere.html
2. http://www.bbc.co.uk/scotland/education/int/geog/rivers/hydrographs/index.shtml
2. http://geobytesgcse.blogspot.com/2006/11/hydrographs-and-river-discharge.html


  1. Hi Xu Ao! :) I learnt a lot about the factors affecting storm hydrographs (this post should have been written before our test to benefit us more hehehehe) :P

    Just a little question that I have on my mind now... The lag time for each type of rain differs; so what type of rain would contribute to a longer lag time? On the other hand, what type of rain would contribute to a shorter lag time?

    Free for all to reply and contribute! Thank you :)

    Clarissa (7)

  2. Hi, Clarissa.

    Heavy rainfall would contribute to a shorter lag time while light or moderate rainfall would contribute to a longer lag time.

    In the event of a heavy rainfall, precipitation would be high. As a result, the rate in which water enters the soil would be higher, causing the soil to be infiltrated to full capacity within a short period of time. This would lead to a greater surface runoff and water would reach the river faster, hence shortening the lag time and increasing the risk of flooding.

    Meanwhile, in the event of a light or moderate rainfall, the rainwater would take a longer time to flow through the drainage basin to reach the river because more water would be absorbed through infiltration, reducing the amount of surface runoff. Since a large amount of water flowing into the soil would be absorbed by plants, less water would reach the river. As such, the lag time would be longer and there would be lower risk of flooding.

    Hope this helps, and do correct me if there's any mistakes! Thanks (:

    Sherye (13)

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