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 ? June 2015 2

Standard Rainfall Distributions June 2015 3

Geographic Application June 2015 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. June 2015 5

Example of a Nested Distribution 0. 6 0. 7 3 hr 6 hr June 2015 6

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

Select State June 2015 8

Selected Ohio June 2015 9

Selected Columbus Ohio June 2015 10

NOAA 14 Rainfall Ratios June 2015 11

Symmetrical Distribution June 2015 12

Compare to Type II June 2015 13

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

New NRCS rainfall distributions June 2015 15

New NRCS rainfall distributions based on NOAA 14 data for SE states June 2015 16

Compare MSE 1 to 6 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, MSE 1, MSE 2, MSE 3, MSE 4, MSE 5, and MSE 6 • Run both the standard NRCS and DMV unit hydrographs. June 2015 17

Compare peak discharges from the rainfall distributions for 484 DUH Tc hours Type II MSE 1 MSE 2 MSE 3 MSE 4 MSE 5 MSE 6 0. 5 1970 2511 2287 2066 1853 1597 1431 0. 75 1517 1940 1775 1623 1467 1282 1154 1. 0 1234 1593 1466 1348 1225 1083 981 1. 5 917 1182 1096 1017 933 839 766 2. 0 736 945 882 825 764 694 637 June 2015 18

Compare peak discharges from the rainfall distributions for DMV DUH Tc hours Type II MSE 1 MSE 2 MSE 3 MSE 4 MSE 5 MSE 6 0. 5 1202 1529 1400 1282 1164 1031 936 0. 75 895 1141 1055 977 896 803 732 1. 0 724 917 853 796 734 663 607 1. 5 532 665 626 587 501 463 2. 0 424 531 497 471 407 379 June 2015 19

Points used to represent Arkansas counties June 2015 20

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Future Directions NOAA Atlas 14 data may be imported easily into Win. TR-20 Version 3. 10. • Use GIS data layers to derive site-specific and regional rainfall distributions for hydrologic modeling. June 2015 23

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