Prepared by Lesley Sweeney
Environmental Engineer, CT NRCS
November, 2012

What is Extreme Precipitation?
• A large precipitation event
• Design storm of a given duration with a
Recurrence (or frequency) of 1yr -500yr
(e.g. 10-year 24-hour rainfall)
• NOT the amount of Annual Rainfall

Recurrence is all about probability…
• 100-yr Storm
1 / 100 = 1% chance of occurrence in any year
• 25-yr Storm
1 / 25 = 4% chance of occurrence in any year
• 2-yr Storm
1 / 2 = 50% chance of occurrence in any year
• 1-yr Storm
1 / 1 = 100% chance of occurrence??? 99.9% chance

Climate Change?
Changes in Extreme Precipitation?
Increases in Amounts of Very Heavy
Precipitation (1958 to 2007)
The map shows percent increases in the amount falling in very heavy precipitation events (defined as the heaviest 1 percent of all daily events) from 1958 to 2007 for each region. There are clear trends toward more very heavy precipitation for the nation as a whole, and particularly in the Northeast and Midwest.
U.S. Global Change Research Program. 2009. Global climate change impacts in the United States: a state of knowledge report . Cambridge [England]: Cambridge
University Press.
DeGaetano, Arthur T., 2009: Time-Dependent
Changes in Extreme-Precipitation Return-
Period Amounts in the Continental United
States.
J. Appl. Meteor. Climatol.
, 48 , 2086–
2099.

History of Extreme Precipitation
Technical Paper 40 (1961) Wilks (1993)
NOAA Atlas 14 (2004)
NRCC (2010)

Northeast Regional Climate Center web site: www.precip.net
Joint collaboration between:
•Northeast Regional Climate Center
(NRCC)
NOAA Regional Climate Centers,
Applied Climate Information
System (ACIS), National Climatic
Data Center (NCDC), and Cornell
University
•Natural Resources Conservation
Service (NRCS)
Maine, Vermont, New Hampshire,
Massachusetts, Rhode Island,
Connecticut, and New York
6

Project Goals
• Create an updated, comprehensive extreme precipitation atlas for the Northeast US.
• Match the products and output of NOAA Atlas 14 for NRCS compatibility.
• Add additional products to supplement NRCS hydrologic design.
• Produce real-time monitoring and climate change tools.

N-minute Hourly
5min
10min
15min
1hr
2hr
3hr
30min
60min
120min
6hr
12hr
24hr
48hr
Scope of Data/Output
Daily
1day
2day
4day
7day
10day
25yr
50yr
100yr
200yr
500yr
Years
1yr
2yr
5yr
10yr
States
Connecticut
Maine
Massachusetts
New Hampshire
New York
Rhode Island
Vermont
(surrounding states)
(Canada)

Replaced Publications used by
NRCS
• Technical Paper 40, 1961. 30-minute to 24-
hour for 1-year to 100-year return periods.
• NWS HYDRO-35, 1977. 5- to 60- Minute
Precipitation Frequency for the Eastern and
Central United States.
• Technical Paper 49, 1964. Two- to Ten- Day
Precipitation for Return Periods of 2 to 100 years.

NRCC vs. TP-40 10-year 24-hour, percent more or less

NRCC vs. TP-40 100-year 24-hour, percent more or less

NRCC – Hydro35 10-year 60-min percent more or less

NRCC vs. Hydro35 100-year 60-min., percent more or less

NRCC vs. TP-49 100-year 10-day, percent more or less

• NRCC data shows general increase as return period gets larger for 60-minute and 24-hour durations.
• NRCC 100-year 10-day precipitation trend is lower on average.

Old Rainfall Distributions

Steps in developing a distribution
• Determine ratios of 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.

Example distribution
30 min 10 min 5 min
0.6
0.7
6 hr
3 hr

Site and Storm Frequency-Specific
Distribution Curves

How does this translate to design runoff in Connecticut?
• Generally for the 1 yr-10 yr 24-hr storm, no large change in runoff.
• For the 50 yr and 100 yr 24-hr storm, likely higher runoff due to higher precipitation amounts .
• The 25 yr 24-hr storm trends towards higher due to increased rainfall amount but may be moderated by the rainfall distribution.

6
5
4
3
2
1
0
Rainfall Distribution Comparison
300
100-ac CN = 65
1-ac CN = 65
250
NE C
NE D
TYPE III
200
150
100
50
0
NE C
NE D
Type III
1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr 1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
25
20
15
10
5
0
40
35
30
10-ac CN = 65
1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr
NE C
NE D
Type III
1600
1400
1200
1000
800
600
400
200
0
1000-ac CN = 65
NE C
NE D
Type III
1-yr 2-yr 5-yr 10-yr 25-yr 50-yr 100-yr

Getting Data from www.precip.net

NRCS Runoff Calculation Tools
• WinTR-20, compatible to import rainfall table and create distribution curves
• WinTR-55
Accepts individual storms and individual distribution curves. 2 Methods to implement the data
• EFH2
County rainfall files and regional distribution curves will be added

Importing NRCC data into WinTR-20

Importing data into WinTR-20

Importing data into WinTR-20

Importing data into WinTR-20
• Select “Import NOAA
–NRCC Data” from
“File” pulldown menu.
• Locate the directory, select text file name
“CT_Windham.txt”

Importing data into WinTR-20

Using NRCC data with WinTR-55
Two Methods
• Using County Data
Typical method using representative county precipitation values developed using the NRCC data and regionalized rainfall distributions
• Using Site Specific Data
Each frequency storm is run separately

County Rainfall for WinTR-55
Rainfall data at the geographical county centroid.

Regional Rainfall Distribution Curves

WinTR-55 Example using County Data

WinTR-55 Example using County Data

Using Site Specific Data in WinTR-55

Using Site Specific Data in WinTR-55

Conclusions
• Precipitation amounts have not changed significantly for 1yr-10 yr 24-hour design storms
• Precipitation amounts have increased for 25 yr and above 24-hour design storms
• New site and storm specific Rainfall Distributions
• New extreme precipitation data can be used with
NRCS programs to determine design storm runoff

References
1.
www.precip.net
Outreach Presentations
2.
Evaluating Runoff Predictions from Rainfall Tables and Generalized Distribution
Curves for EFH-2 and WinTR-20 in New York State, Paper Number: 121338007,
Peter Wright State Conservation Engineer USDA NRCS NY
3.
Design Rainfall Distributions Based on NOAA Atlas 14 Powerpoint, Geoffrey
Cerrelli, P.E. Hydraulic Engineer USDA NRCS PA, ASABE – 2010 Conference.
4.
Using Northeast Regional Climate Center Extreme Precipitation Data with
WinTR-55 in Connecticut, Ben Smith, USDA NRCS CT, May 2012

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Slide 46

NOTICE TO HISPANIC OR WOMAN FARMERS
If you are a woman or Hispanic farmer and you believe you were denied farm loan benefits by USDA between 1981 and 2000, you may be eligible for compensation.
For more information, call 1-888-508-4429 or visit www.farmerclaims.gov.
Information about USDA civil rights cases can be found online at: a. Hispanic or Women Farmers www.farmersclaims.gov or call 1-888-508-4429 b. Black Famers www.blackfarmercase.com or call 1-888-950-5547.
c. Native American Farmers www.IndianFarmClass.com or call 1-888-233-5506.

Questions?
Lesley Sweeney
Environmental Engineer
860-429-1084
Lesley.Sweeney@ct.usda.gov