CWR4101: Hydrology

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CWR4101 Hydrology
Mr. Jeff Earhart earhart_at_bhiinc.com 407-426-8994 O
ffice hours By Appointment http/classes.cecs.uc
f.edu/cwr4101/shagen
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Rainfall DistributionAppendix C
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Rainfall Distribution
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IDF Curves

• Intensity-Duration-Frequency Curves
• Statistical relationship
• Average intensity vs. rainfall duration
• Ideally developed for calculating flow rates

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Rainfall Maps

• Figure 3.13
• SFWMD
• Rainfall Atlas

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Average Watershed Precipitation

• There several methods to estimate average
  precipitation for a given region
• Arithmetic
• Thiessen
• Isohyetal
• Reciprocal-Distance-Squared

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Arithmetic Average Precipitation

• The Arithmetic Method uses only those gaging
  stations within the watershed area.
• P ?Pi/n
• where Pi Average precipitation over cell
• n gages within the watershed.

n
i 1
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Arithmetic Average Precipitation
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Arithmetic Average Precipitation

• Given that
• P1 3 inches
• P2 1.5 inches
• P3 0.75 inches
• P4 2.25 inches
• P P1 P2 P3/3
• P (3 1.5 0.75)/3 1.75
  inches

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Thiessen Method

• The Thiessen method adjusts for the nonuniform
  location of gaging stations by attempting to
  determine the area of influence.
• The procedure consists of connecting each
  precipitation station with straight lines and
  constructing perpendicular bisectors of the
  connecting lines to form polygons with these
  bisectors.

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Thiessen Method

• n
• P ?WiPi
• i 1
• where Wi Ap/A
• Ap Area of the polygon
• A Total area
• Pi average precipitation over cell.

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Thiessen Method
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Thiessen Method
P1
A1
A2
P2

A3
A4
P3
P4
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Isohyetal Method

• The Isohyetal method is an accurate method to
  estimate average precipitation within a
  topographic basin however, this average is
  difficult to reproduce because of the complexity
  of drawing isohyets.

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Isohyetal average Precipitation

• n
• P ?WiPi
• i 1
• where Wi Ai/A
• Ai Area of cell
• A Total area
• Pi average precipitation over cell
• n total number of cells.

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Isohyetal Method

P1 3.0 inches P2 1.5 inches P3 0.75
inches P4 2.25 inches
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Reciprocal Distance Squared (RDS) Method

• This method is generally used because of its
  reproduction and accuracy to estimate average
  precipitation over a watershed area.

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R-D-S Method

• Procedure
• Divide watershed into grid boxes, Gi
• For each grid box center
• Record GiPj
• Sum the reciprocal of these distances squared
• Ri?1/(GiPj)2

n
i1
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R-D-S Method

• 3. Compute the weighted contribution from each
  station to each grid box
• Pi Pj1/(GiPj)2/Ri
• 4. Compute the overall average precipitation
• P ?Pi/i

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R-D-S Method
G1

• EXAMPLE
• Given that
• G1P1 0.75 units
• G1P2 0.75 units
• G2P1 0.75 units
• G2P2 0.75 units

.
.
P2
P1
G2
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R-D-S Method

• R1 1/(G1P1)2 1/(G1P2)2
• R1 1/0.752 1/0.752 2/0.752
• R1 R2 2/0.752
• P1 P1 (1/0.752)/(2/0.752) P2
  (1/0.752)/(2/0.752)
• P1 P2 0.5P1 0.5P2
• 3. P ?Pi/i 0.5P1 0.5P2

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R-D-S Method
P1
P2

P3
P4