Design Rainfall Distributions Based on NOAA Atlas 14

Design Rainfall Distributions Based on NOAA Atlas 14 Win. TR-20 NOAA 14 Data March 2009

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 ? Win. TR-20 NOAA 14 Data March 2009 2

Standard Rainfall Distributions Win. TR-20 NOAA 14 Data March 2009 3

Geographic Application Win. TR-20 NOAA 14 Data March 2009 4

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. Win. TR-20 NOAA 14 Data March 2009 5

Example of a Nested Distribution 0. 6 0. 7 3 hr 6 hr Win. TR-20 NOAA 14 Data March 2009 6

NOAA Atlas 14 web site • Download rainfall frequency data at: • http: //hdsc. nws. noaa. gov/ Win. TR-20 NOAA 14 Data March 2009 7

Select State Win. TR-20 NOAA 14 Data March 2009 8

Selected Ohio Win. TR-20 NOAA 14 Data March 2009 9

Selected Columbus Ohio Win. TR-20 NOAA 14 Data March 2009 10

NOAA 14 Rainfall Ratios Win. TR-20 NOAA 14 Data March 2009 11

Symmetrical Distribution Win. TR-20 NOAA 14 Data March 2009 12

Compare to Type II Win. TR-20 NOAA 14 Data March 2009 13

Where Type II and Type III compare with NOAAderived distribution, 25 -yr rainfall • Purple: Type II • Pink: Type III • Green: both Win. TR-20 NOAA 14 Data March 2009 14

Points used to represent Maryland Counties Win. TR-20 NOAA 14 Data March 2009 15

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Win. TR-20 NOAA 14 Data March 2009 17

Future Directions Develop software to easily use the data. • Use GIS data layers to derive site-specific rainfall data for hydrologic modeling. Win. TR-20 NOAA 14 Data March 2009 18

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