Using NOAA Atlas 2 to derive a full depth-duration-frequency table

NOAA Atlas 2 was written for the Soil Conservation Service (now the Natural Resources Conservation Service) by the National Weather Service to determine the 6 and 24-hour duration frequencies for 2 to 100 year rainfall events. These are the most common values required by NRCS design procedures.

THE EQUATIONS GIVEN HERE ARE FOR IDAHO ONLY!! OTHER STATES USE A SIMILAR PROCEDURE BUT DIFFERENT EQUATIONS
 
 The maps can be found in the Climate Lab (EP 425) or at the Western Regional Climate Center

Many times, values for durations other than those in the Atlas are required such as in events with durations shorter than six hours. The narrative in the Atlas gives procedures for deriving such values. The formulas given here are from the Idaho volume. With these, you may derive depth-duration-frequency or intensity-duration-frequency combinations for durations from 5 minutes to 24 hours. An Excel spreadsheet that does these calculations for you is available. You need only enter the location and all the 6 and 24 hour duration values as asked for on the NOAA_Atlas_2 sheet. The spreadsheet will calculate all the other durations and the intensity-duration-frequency curve for you.

There are also several spreadsheets where the IDF curves have already been calculated. If you can use one of these, you do not need to have access to the NOAA2 Atlas.

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A description of the procedure, taken from NOAA Atlas2, is given below:

In the notation below, P_6,2 means the 6 hour-2 year precipitation frequency. Likewise, P_24,50 is the 24 hour-50 year precipitation frequency

1.Determine the latitude, longitude and elevation (in feet) of the location.

2.Determine the depths for the 6 and 24 hour durations for frequencies from 2 to 100 years.

3.Determine the region in which the required point is located. Idaho is divided into three regions as shown on the map in the Atlas. The regions are:

Region 1:The Snake River Plain below a smoothed contour of 5000 feet.
Region 2:Mountainous region of Idaho and Montana extending to the Continental Divide.
Region 3:The Great Basin area in southeastern Idaho.

4.Obtain the 1-hour duration values for the 2 and 100-year frequencies.

Region 1: P_1,2 = 0.077 + 0.715A - 0.0004X₅X₆
P_1,100 = 0.187 + 0.833B

Region 2: P_1,2 = 0.019 + 0.711A - 0.001Z
P_1,100 = 0.338 + 0.670B + 0.001Z

Region 3: P_1,2 = 0.005 + 0.852A
P_1,100 = 0.332 + 0.789B

where A = (P_6,2)²/P_24,2 and B = (P_6,100)²/P_24,100

X₅ = Latitude - 40 in decimal degrees (47° 45' = 7.75)
X₆ = Longitude - 100 in decimal degrees (116° 45' = 16.75)
Z = Elevation in hundreds ( 4580 ft = 46)

5.To fill out the remainder of the 1-hour duration values for the 5-50 year frequencies, use the following formulas derived from Figure 6:

P_1,f = P_1,2 + a(P_1,100 - P_1,2 )
where:
 

frequency(yrs)	5	10	25	50
a	0.242	0.398	0.613	0.799

6.The 12-hour duration depths for all frequencies are :

P_12,f = P_6,f + 0.5(P_24,f - P_6,f)for f = 2, 5, 10, 25, 50, 100
 

7.The 2 and 3-hour duration values are:

P_d,f = P_1,f + a(P_6,f - P_1,f)

where a is determined from the following table:
 

	Region
Duration(hrs)	1	2	3
2	0.278	0.250	0.299
3	0.503	0.468	0.526

8.For durations of 5, 10, 15, and 30 minutes, the following factors are used for all frequencies:

P_d,f = aP_1,f

where
 

duration(minutes)	5	10	15	30
a	0.29	0.45	0.57	0.79

9.The values below were determined for a location at 5600 feet, latitude 48° 45' and longitude of 116° 30'
 

	Frequency (years)
Duration	2	5	10	25	50	100
(Minutes)
5	0.13	0.18	0.22	0.26	0.3	0.35
10	0.2	0.28	0.33	0.41	0.47	0.54
15	0.25	0.35	0.42	0.52	0.6	0.69
30	0.34	0.49	0.59	0.72	0.83	0.95
(Hours)
1	0.43	0.62	0.74	0.91	1.05	1.21
2	0.55	0.77	0.89	1.08	1.23	1.40
3	0.65	0.89	1.06	1.23	1.38	1.58
6	0.90	1.20	1.35	1.60	1.75	2.00
12	1.02	1.31	1.57	1.85	20.3	2.25
24	1.60	2.00	2.40	2.80	3.00	3.33

3/93 updated 1/98, 2/99