1. Design Rainfall Distributions Based on NOAA Atlas 14

2. June 2015 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 ? 2

3. June 2015 Standard Rainfall Distributions 3

4. June 2015 Geographic Application 4

5. June 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. 5

6. June 2015 Example of a Nested Distribution 3 hr 0.6 6 hr 0.7 6

7. June 2015 NOAA Atlas 14 web site Download rainfall frequency data at: http://hdsc.nws.noaa.gov/hdsc/pfds/ 7

8. Select State June 2015 8

9. Selected Ohio June 2015 9

10. Selected Columbus Ohio 10

11. June 2015 NOAA 14 Rainfall Ratios 11

12. June 2015 Symmetrical Distribution 12

13. June 2015 Compare to Type II 13

14. June 2015 Where Type II and Type III compare with NOAA- derived distribution, 25-yr rainfall Purple: Type II Pink: Type III Green: both White: Neither 14

15. June 2015 NRCS Rainfall Distribution Regions 15

16. June 2015 Development of NOAA_A, B, C, and D rainfall distributions Compare to the 60-min / 24-hr ratio for the Type II of 0.45 and Type III of 0.40 16 Rainfall DistributionMinimum 60-min / 24-hr ratio Maximum 60-min / 24-hr ratio NOAA_A 0.48 0.58 NOAA_B 0.43 0.48 NOAA_C 0.38 0.43 NOAA_D 0.28 0.38

17. June 2015 Compare NOAA_A, B, C, and D rainfall distributions to Type II Drainage Area = 1.0 sq mi. CN = 77 Tc = 0.5 to 2.0 hours Rainfall = 6.0 inches in 24 hours Run Type II, NOAA_A, NOAA_B, NOAA_C, and NOAA_D Run both the standard NRCS and DMV unit hydrographs. 17

18. June 2015 Compare peak discharges from the rainfall distributions for 484 DUH 18 Tc hoursType IINOAA_ANOAA_BNOAA_CNOAA_D 0.519702008177315901430 0.7515171586141712791153 1.01234132711961084981 1.59171011925843769 2.0736824760697642

19. June 2015 Compare peak discharges from the rainfall distributions for DMV DUH 19 Tc hoursType IINOAA_ANOAA_BNOAA_CNOAA_D 0.51202126111341029936 0.75895961878804731 1.0724790724665611 1.5532586544503467 2.0424470439409383

20. June 2015 Points used to represent Maryland counties and DC 20

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23. June 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. 23

24. June 2015 24