EXTREME PRECIPITATION 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? DeGaetano, Arthur T., 2009: Time-Dependent Changes in Extreme-Precipitation Return- Period Amounts in the Continental United States. J. Appl. Meteor. Climatol., 48, 2086– The map shows percent increases in the amount falling in very heavy precipitation events (defined as the heavi- est 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 par- ticularly in the Northeast and Midwest. Increases in Amounts of Very Heavy Precipitation (1958 to 2007) U.S. Global Change Research Program Global climate change impacts in the United States: a state of knowledge report. Cambridge [England]: Cambridge University Press. Changes in Extreme Precipitation?

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

6 Northeast Regional Climate Center web site: 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

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.

Scope of Data/Output N-minuteHourlyDaily 5min1hr1day 10min2hr2day 15min3hr4day 30min6hr7day 60min12hr10day 120min24hr 48hr Years 1yr 2yr 5yr 10yr 25yr 50yr 100yr 200yr 500yr States Connecticut Maine Massachusetts New Hampshire New York Rhode Island Vermont (surrounding states) (Canada)

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

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

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

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

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

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

Precipitation Trends 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 3 hr hr min 10 min 30 min

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.

Rainfall Distribution Comparison

Getting Data from

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

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 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 Two Methods

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

Regional Rainfall Distribution Curves

WinTR-55 Example using County Data

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. Outreach Presentationswww.precip.net 2.Evaluating Runoff Predictions from Rainfall Tables and Generalized Distribution Curves for EFH-2 and WinTR-20 in New York State, Paper Number: , 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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Questions? Lesley Sweeney Environmental Engineer