Lecture 23 Runoff (3) Runoff Temporal Variation

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Lecture 23 Runoff (3)Runoff Temporal Variation

• Quick and delayed flow separation
• Daily variations
• Seasonal variations
• Long-term variations

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Hydrographs separation into quick and delayed
flows
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River bank storage and influence
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Daily Flow Variations

• Large rivers show small daily discharge
  variations weekly flow values are suitable
• Small rivers show relatively large daily
  discharge variations daily flow values are
  needed to characterize the variation.

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Seasonal Flow Variations

• Snowmelt is most significant for discharge near
  glaciers
• Evapotranspiration reduces river flow in summer
  months
• These two processes determine the seasonal
  patterns at various locations on the same river

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Long-term Variations

• Marked differences in long-term hydrograph
  variation patterns between wet and dry regions
  dry regions are more variable
• Persistent dry and wet periods are often found
  in dry regions
• Long-term hydrograph means may also vary with
  time and depend on the length of records

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Long-term Variations

• Marked differences in long-term hydrograph
  variation patterns between wet and dry regions
  dry regions are more variable
• Persistent dry and wet periods are often found
  in dry regions
• Long-term hydrograph means may also vary with
  time and depend on the length of records

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Long-term Variation in Discharge
If climate has not changed and the rainfall
record is long enough, we expect to find no
trends with time. But in many cases we find
long-term trends climate may be changing.
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Extreme Events
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Terms to Remember
Annual maximum discharge usually means the
maximum discharge in a day of a year because
daily discharge has significance in many
water-related engineering projects Recurrence
interval, normally in years, is the interval of
the occurrence of annual maximum discharge of
certain value Exceedance probability is the
probability that daily discharge exceeds a
certain rate.
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Extreme Events