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Design Rainfall Distributions
Based on NOAA Atlas 14 in Maryland
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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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June 2015
Standard Rainfall Distributions
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June 2015
Geographic Application
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June 2015
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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June 2015
Example of a Nested Distribution
3 hr
0.6
6 hr
0.7
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June 2015
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
June 20159
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June 2015
Selected Columbus Ohio
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June 2015
NOAA 14 Rainfall Ratios
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June 2015
Symmetrical Distribution
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June 2015
Compare to Type II
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June 2015
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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June 2015
New NRCS rainfall distributions based on NOAA 14 data for Maryland
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June 2015
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
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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
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June 2015
Points used to represent Maryland counties
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June 2015
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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