appendix b of the tr-55 manual

B–1(210-VI-TR-55, Second Ed., June 1986)

Appendix B Synthetic Rainfall Distributions andRainfall Data Sources

The highest peak discharges from small watersheds inthe United States are usually caused by intense, briefrainfalls that may occur as distinct events or as part ofa longer storm. These intense rainstorms do not usu-ally extended over a large area and intensities varygreatly. One common practice in rainfall-runoff analy-sis is to develop a synthetic rainfall distribution to usein lieu of actual storm events. This distribution in-cludes maximum rainfall intensities for the selecteddesign frequency arranged in a sequence that is criticalfor producing peak runoff.

Synthetic rainfall distributions

The length of the most intense rainfall period contrib-uting to the peak runoff rate is related to the time ofconcentration (Tc) for the watershed. In a hydrographcreated with NRCS procedures, the duration of rainfallthat directly contributes to the peak is about 170percent of the Tc. For example, the most intense 8.5-minute rainfall period would contribute to the peakdischarge for a watershed with a Tc of 5 minutes. Themost intense 8.5-hour period would contribute to thepeak for a watershed with a 5-hour Tc.

Different rainfall distributions can be developed foreach of these watersheds to emphasize the criticalrainfall duration for the peak discharges. However, toavoid the use of a different set of rainfall intensities foreach drainage area size, a set of synthetic rainfalldistributions having “nested” rainfall intensities wasdeveloped. The set “maximizes” the rainfall intensitiesby incorporating selected short duration intensitieswithin those needed for longer durations at the sameprobability level.

For the size of the drainage areas for which NRCSusually provides assistance, a storm period of 24 hourswas chosen the synthetic rainfall distributions. The 24-hour storm, while longer than that needed to deter-mine peaks for these drainage areas, is appropriate fordetermining runoff volumes. Therefore, a single stormduration and associated synthetic rainfall distributioncan be used to represent not only the peak dischargesbut also the runoff volumes for a range of drainagearea sizes.

The intensity of rainfall varies considerably during astorm as well as geographic regions. To representvarious regions of the United States, NRCS developedfour synthetic 24-hour rainfall distributions (I, IA, II,and III) from available National Weather Service(NWS) duration-frequency data (Hershfield 1061;Frederick et al., 1977) or local storm data. Type IA isthe least intense and type II the most intense shortduration rainfall. The four distributions are shown infigure B-1, and figure B-2 shows their approximategeographic boundaries.

Types I and IA represent the Pacific maritime climatewith wet winters and dry summers. Type III representsGulf of Mexico and Atlantic coastal areas where tropi-cal storms bring large 24-hour rainfall amounts. TypeII represents the rest of the country. For more precisedistribution boundaries in a state having more thanone type, contact the NRCS State Conservation Engi-neer.

0 3 6 9 12 15 18 21 24

1.0

0.5

0.0

IA

I

II

III

Time (hours)F

rac

tio

n o

f2

4-h

ou

r ra

infa

ll

Figure B-1 SCS 24-hour rainfall distributions

B–2 (210-VI-TR-55, Second Ed., June 1986)

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Type I

Type IA

Type II

Type IIIIII

III

III

I

IA

II

III

Rainfall

Distribution

Figure B-2 Approximate geographic boundaries for NRCS (SCS) rainfall distributions

Rainfall data sourcesThis section lists the most current 24-hour rainfall datapublished by the National Weather Service (NWS) forvarious parts of the country. Because NWS TechnicalPaper 40 (TP-40) is out of print, the 24-hour rainfallmaps for areas east of the 105th meridian are includedhere as figures B-3 through B-8. For the area generallywest of the 105th meridian, TP-40 has been supersededby NOAA Atlas 2, the Precipitation-Frequency Atlas ofthe Western United States, published by the NationalOcean and Atmospheric Administration.

East of 105th meridian

Hershfield, D.M. 1961. Rainfall frequency atlas of theUnited States for durations from 30 minutes to 24hours and return periods from 1 to 100 years. U.S.Dept. Commerce, Weather Bur. Tech. Pap. No. 40.Washington, DC. 155 p.

West of 105th meridian

Miller, J.F., R.H. Frederick, and R.J. Tracey. 1973.Precipitation-frequency atlas of the Western UnitedStates. Vol. I Montana; Vol. II, Wyoming; Vol III, Colo-rado; Vol. IV, New Mexico; Vol V, Idaho; Vol. VI, Utah;Vol. VII, Nevada; Vol. VIII, Arizona; Vol. IX, Washing-ton; Vol. X, Oregon; Vol. XI, California. U.S. Dept. of

Commerce, National Weather Service, NOAA Atlas 2.Silver Spring, MD.

Alaska

Miller, John F. 1963. Probable maximum precipitationand rainfall-frequency data for Alaska for areas to 400square miles, durations to 24 hours and return periodsfrom 1 to 100 years. U.S. Dept. of Commerce, WeatherBur. Tech. Pap. No. 47. Washington, DC. 69 p.

Hawaii

Weather Bureau. 1962. Rainfall-frequency atlas of theHawaiian Islands for areas to 200 square miles, dura-tions to 24 hours and return periods from 1 to 100years. U.S. Dept. Commerce, Weather Bur. Tech. Pap.No. 43. Washington, DC. 60 p.

Puerto Rico and Virgin Islands

Weather Bureau. 1961. Generalized estimates of prob-able maximum precipitation and rainfall-frequencydata for Puerto Rico and Virgin Islands for areas to 400square miles, durations to 24 hours, and return periodsfrom 1 to 100 years. U.S. Dept. Commerce, WeatherBur. Tech. Pap. No. 42. Washington, DC. 94 P.


Figure B-3 2-year, 24-hr rainfall

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0 200 600 Mi

Rainfall iso-line

Rainfall iso-line half unit

Rainfall iso-line (depression)

Albers Equal Area Projection

400

2-Year 24-Hour Rainfall (inches)

1.5

2.0

2.53.0

3.5

4.0 4.5

5.05.5 6.0

6.05.5

5.0

4.55.5

6.06.05.5

5.0

5.0

5.0

4.54.0

4.0

3.5

3.0

2.5

3.0

3.5

4.03.5

5.0

2.0

2.0

2.0

2.5

3.0

3.5

4.0

4.5

2.0USE NOAA ATLAS 2 MAPS FOR

WESTERN STATES

Figure B-4 5-year, 24-hour rainfall

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5–Year 24–Hour Rainfall (inches)

USE NOAA ATLAS 2 MAPS FOR

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7

6.5

6

6.57

7.58

6.5

65.5

5

4.5

4.55

4.55

4

4 3.5

3

2.5

2.5

2.5

3

2.5

3

3.5

4

4.5

3

5

5.5

6

6.57

2.5

56

33.5

44.5

5

5.56

6.5

77.5

8 8.5

8.5 87.5

7

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25-Year 24- Hour Rainfall (inches)

7

8

67

66.5

6

6

8.5

9

10

11

11

1099

810

101112

11

10

98

7

65

4

3.5

43.5

3.5

4

3.5

3.55

6

7

8

9

7 89

4

5


WESTERN STATES


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3

3.5

45

6

7

8

910

109

87

8

8

9

9

6

7

5

4

3.5

5

6

6

67

3

3

3.5

3

3


WESTERN STATES

3.5

4

5

6

7

8




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6

6

7

5

4.5

6

7

8

9

10

11

78 8

910

10

1112

11

10

10

10

11

12

13

1211

109

8

7654.5

43.5

5

4.54

4

4.5

5

4

7

89


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8

9

76

5

4

10

11

12 13

1415

15 1413

12

1111

13

14

1413

12

11

11

1110

9

8

7

6

5

7

8

89

89

10

5

6

5 7

8

9

10

11

12

54

13

4

4

400

10


WESTERN STATES

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appendix b of the tr-55 manual

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