Surface Water Hydrology and Watersheds

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Surface Water Hydrology and Watersheds

• Ismail KILINÇ
• Istanbul Technical University
• December, 2004

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The Hydrologic Cycle
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Major Hydrologic Processes

• Precipitation (measured at rain gage)
• Evaporation or ET (loss to atmosphere)
• Infiltration (loss to subsurface)
• Overland Flow
• Stream Flow (measured at stage gage)
• Ground Water Flow

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The Watershed or Basin

• Area of land that drains to a single outlet and
  is separated from other watersheds by a drainage
  divide.
• Rainfall that falls in a watershed will create
  runoff to that watershed outlet.
• All other rainfall falling outside a basin will
  not affect the runoff response.

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Sample Watershed
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The Hyetograph

• Graph of Rainfall Rate vs Time at a Single Gage
• Usually Plotted as a Bar Chart
• Net Rainfall is Found by Subtr. Infiltration
  Losses
• Integration of Net Rainfall in Time yields the
  Total Rainfall Vol (DRO) in inches over a
  Watershed

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The Hydrograph

• Graph of Discharge vs Time at a Single Location
• Rising Limb, Crest Segment, Falling Limb,and
  Recession
• Base Flow is Usually Subtracted to yield DRO
• Peak Gives the Maximum Flow for the Event

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• Hyetograph
• Watershed
• The Hydrograph

Q
T
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Rainfall and Runoff Response
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Unit Hydrograph Theory

• The unit hydrograph represents the basin response
  to 1 cm of uniform net rainfall for a specified
  duration.
• Linear method originally devised in 1932.
• Works best for relatively small subareas.
• Several computational methods exist.

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Methods of Unit Hydrograph

• From streamflow observations
• Synthetically
• Geomorphical methods

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From streamflow data

• Measured at an
• observing station
• or elevation above
• a specific datum
• in a channel

(From Stream-Gaging Program of the U.S.
Geological Survey U.S. GEOLOGICAL SURVEY
CIRCULAR 1123 Reston, Virginia, 1995 By Kenneth
L. Wahl, Wilbert O. Thomas, Jr., and Robert M.
Hirsch )
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From streamflow data
(From Stream-Gaging Program of the U.S.
Geological Survey U.S. GEOLOGICAL SURVEY
CIRCULAR 1123 Reston, Virginia, 1995 By Kenneth
L. Wahl, Wilbert O. Thomas, Jr., and Robert M.
Hirsch )
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• Magnitude, shape and timing of hydrograph are
  also dependent on
• the characteristics of the watershed
• size ? slope
• shape ? storage
• the rainfall event
• intensity ? duration

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Synthetic Unit Hydrograph Methods

• Snyders Method (1938)
• Clark TC R Method (1945)
• Nash (1958)
• SCS Method (1964)

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Clark TC R Method

• The processes of translation and attenuation
  dominate the movement of flow through a
  watershed.

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Clark UnitHydrograph Computation
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Surface Runoff Theory

• TC Travel time of overland runoff from most
  remote point to the outlet
• R Routing Coefficient Relates Storage and
  Outflow

where L channel length (mi) S slope of
channel (ft/mi) LCA length to centroid (mi) D
0.94 for developed watersheds of Solt20
ft/mi So watershed slope (ft/mi)
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• Thank you