design rainfall distributions based on noaa atlas 14 in maryland

Design Rainfall Distributions

Based on NOAA Atlas 14 in Maryland

June 2015

General Considerations

• Standard NRCS rainfall distributions were developed between 1960 and 1985.

• NRCS is adopting NOAA Atlas 14 in the states where it is applicable.

• Will a new set of rainfall distributions be required ?

• If so, how to develop and apply them ?

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Standard Rainfall Distributions

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Geographic Application

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Steps in developing a distribution

• Determine ratios of x hour / 24 hour rainfall.

• Place the rainfall ratio for the shortest duration in the center of the distribution.

• Symmetrically place each larger duration to include the shorter durations.

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Example of a Nested Distribution

3 hr

0.6

6 hr

0.7

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NOAA Atlas 14 web site

• Download rainfall frequency data at:

• http://hdsc.nws.noaa.gov/hdsc/pfds/

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Select State

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Selected Ohio

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Selected Columbus Ohio

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NOAA 14 Rainfall Ratios

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Symmetrical Distribution

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Compare to Type II

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Where Type II and Type III compare with NOAA-derived distribution, 25-yr rainfall

• Purple: Type II

• Pink: Type III

• Green: both

• White: Neither

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New NRCS rainfall distributions based on NOAA 14 data for Maryland

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Compare new rainfall distributions to Type II for Maryland

• Drainage Area = 1.0 sq mi.• CN = 77• Tc = 0.5 to 2.0 hours• Rainfall = 6.0 inches in 24 hours• Run Type II, NOAA_A, NOAA_B,

NOAA_C, and NOAA_D• Run both the standard NRCS and DMV

unit hydrographs.

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June 2015

Compare peak discharges from the rainfall distributions for 484 DUH

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Tc hours Type II NOAA_A NOAA_B NOAA_C

0.5 1970 2008 1773 1590

0.75 1517 1586 1417 1279

1.0 1234 1327 1196 1084

1.5 917 1011 925 843

2.0 736 824 760 697

June 2015

Compare peak discharges from the rainfall distributions for DMV DUH

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Tc hours Type II NOAA_C NOAA_D

0.5 1202 1029 936

0.75 895 804 731

1.0 724 665 611

1.5 532 503 467

2.0 424 409 383

June 2015

Points used to represent Maryland counties

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Future Directions

NOAA Atlas 14 data may be imported easily into WinTR-20 Version 3.10.

• Use GIS data layers to derive site-specific and regional rainfall distributions for hydrologic modeling.

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