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**Design Rainfall Distributions Based on NOAA Atlas 14**

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**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 WinTR-20 NOAA 14 Data March 2009

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**Standard Rainfall Distributions**

WinTR-20 NOAA 14 Data March 2009

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**Geographic Application**

WinTR-20 NOAA 14 Data March 2009

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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. WinTR-20 NOAA 14 Data March 2009

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**Example of a Nested Distribution**

6 hr 0.7 0.6 3 hr WinTR-20 NOAA 14 Data March 2009

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**NOAA Atlas 14 web site Download rainfall frequency data at:**

WinTR-20 NOAA 14 Data March 2009

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**Select State WinTR-20 NOAA 14 Data March 2009**

Purtto Rico, the Virgin Islands, and Hawaii now have updated data as well as the areas shown in purple on this map. WinTR-20 NOAA 14 Data March 2009

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Selected Ohio WinTR-20 NOAA 14 Data March 2009

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**Selected Columbus Ohio**

WinTR-20 NOAA 14 Data March 2009

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NOAA 14 Rainfall Ratios WinTR-20 NOAA 14 Data March 2009

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**Symmetrical Distribution**

WinTR-20 NOAA 14 Data March 2009

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**Compare to Type II WinTR-20 NOAA 14 Data March 2009**

At this location, the distribution based on NOAA 14 data is more intense than the Type 2. WinTR-20 NOAA 14 Data March 2009

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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 WinTR-20 NOAA 14 Data March 2009

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**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. WinTR-20 NOAA 14 Data March 2009

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WinTR-20 NOAA 14 Data March 2009

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WinTR-20 NOAA 14 Data March 2009

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**Future Directions Develop software to easily use the data.**

Use GIS data layers to derive site-specific rainfall data for hydrologic modeling. WinTR-20 NOAA 14 Data March 2009

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WinTR-20 NOAA 14 Data March 2009

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