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. February 2015
Standard Rainfall Distributions February 2015
Geographic Application February 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. February 2015
Example of a Nested Distribution 6 hr 0.7 0.6 3 hr February 2015
NOAA Atlas 14 web site Download rainfall frequency data at: http://hdsc.nws.noaa.gov/ February 2015
Select State Purtto Rico, the Virgin Islands, and Hawaii now have updated data as well as the areas shown in purple on this map. February 2015
Selected Ohio February 2015
Selected Columbus Ohio February 2015
NOAA 14 Rainfall Ratios February 2015
Symmetrical Distribution February 2015
Compare to Type II February 2015 At this location, the distribution based on NOAA 14 data is more intense than the Type 2. February 2015
Where Type II and Type III compare with NOAA-derived distribution, 25-yr rainfall Purple: Type II Pink: Type III Green: both February 2015
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. February 2015
February 2015
February 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. February 2015
February 2015