Design Rainfall Distributions Based on NRCC Data General

Design Rainfall Distributions Based on NRCC Data

General Considerations • Standard NRCS rainfall distributions were developed between 1960 and 1985. • NRCS is adopting NRCC rainfall data in the states where it is applicable. • Will a new set of rainfall distributions be required ? • If so, how to develop and apply them ? NRCC Rainfall Distributions October 2015 2

Standard Rainfall Distributions NRCC Rainfall Distributions October 2015 3

Geographic Application NRCC Rainfall Distributions October 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. NRCC Rainfall Distributions October 2015 5

Example of a Nested Distribution 0. 6 0. 7 3 hr 6 hr NRCC Rainfall Distributions October 2015 6

NRCC web site • Download rainfall frequency data at: • http: //www. precip. net/ NRCC Rainfall Distributions October 2015 7

Select Data & Products Tab NRCC Rainfall Distributions October 2015 8

Select GIS Data Files • Select 60 -min 25 -yr layer • Select 24 -hr 25 -yr layer NRCC Rainfall Distributions October 2015 9

Prepare GIS layers 24 -hour 25 -year precipitation 60 -minute 25 -year precipitation NRCC Rainfall Distributions October 2015 10

Compute 60 -min / 24 -hr Ratio NRCC Rainfall Distributions October 2015 11

Ratio for each Distribution Region NRCC Rainfall Distributions October 2015 12

Four Rainfall Distribution Regions NRCC Rainfall Distributions October 2015 13

Unsmoothed ratio for each duration based on 25 -yr rainfall NRCC Rainfall Distributions October 2015 14

Smoothed ratio for each duration based on 25 -yr rainfall NRCC Rainfall Distributions October 2015 15

Final Rainfall Distribution Map NRCC Rainfall Distributions October 2015 16

Rainfall Distribution Plots NRCC Rainfall Distributions October 2015 17

Future Directions NRCC data may be imported into Win. TR 20 Version 3. 10. • When NOAA Atlas 14 for New York and New England states is completed, NOAA Atlas 14 will be adopted by NRCS. Most likely, new rainfall distributions and maps will be developed. NRCC Rainfall Distributions October 2015 18

NRCC Rainfall Distributions October 2015 19