Design Rainfall Distributions Based on NOAA Atlas 14

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 1985-86. WinTR-20 NOAA 14 Data March 2009

Standard Rainfall Distributions WinTR-20 NOAA 14 Data March 2009

Geographic Application WinTR-20 NOAA 14 Data March 2009

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

Example of a Nested Distribution 6 hr 0.7 0.6 3 hr WinTR-20 NOAA 14 Data March 2009

NOAA Atlas 14 web site Download rainfall frequency data at: http://hdsc.nws.noaa.gov/ WinTR-20 NOAA 14 Data March 2009

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

Selected Ohio WinTR-20 NOAA 14 Data March 2009

Selected Columbus Ohio WinTR-20 NOAA 14 Data March 2009

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

Symmetrical Distribution WinTR-20 NOAA 14 Data March 2009

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

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

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

WinTR-20 NOAA 14 Data March 2009

WinTR-20 NOAA 14 Data March 2009

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

WinTR-20 NOAA 14 Data March 2009