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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