1. Design Rainfall Distributions Based on NOAA Atlas 14

2. 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 ?
Types I, II, and IA were developed in 1960’s. Type III was developed in
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3. Standard Rainfall Distributions
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4. Geographic Application
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5. 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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6. Example of a Nested Distribution
6 hr
0.7
0.6
3 hr
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7. NOAA Atlas 14 web site Download rainfall frequency data at:
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8. Select State
Purtto Rico, the Virgin Islands, and Hawaii now have updated data as well as the areas shown in purple on this map.
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9. Selected Ohio
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10. Selected Columbus Ohio
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11. NOAA 14 Rainfall Ratios
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12. Symmetrical Distribution
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13. Compare to Type II February 2015
At this location, the distribution based on NOAA 14 data is more intense than the Type 2.
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14. Where Type II and Type III compare with NOAA-derived distribution, 25-yr rainfall
Purple: Type II
Pink: Type III
Green: both
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15. Points used to represent Maryland Counties
100-year 24-hour rainfall was averaged for each county and a point selected with that rainfall. At the latitude and longitude of each point a NOAA 14 text file was downloaded. Washington and Frederick Counties were split in two parts due to differences in rainfall. The background of this slide is the 100-year 24-hour rainfall GIS grid layer downloaded from the NOAA 14 web site.
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18. 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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