Appendix B. DEPTH-DURATION FREQUENCY AND INTENSITY-DURATION FREQUENCY FOR AUSTIN AND TRAVIS COUNTY, TEXAS

November 21, 2001

William H. Asquith
U.S. Geological Survey
8027 Exchange Drive
Austin, Texas 78754

ABSTRACT

The depth-duration frequency (DDF) and intensity-duration frequency (IDF) of annual maxima precipitation were computed for Tom Miller Dam, which is located on the Colorado River in Austin, Travis County, Texas. The computations were based on the data and procedures defined by Asquith (1998). The location of the dam was chosen because it is proximate to central Austin, central Travis County, and is geographically familiar to the general public. Within the implied uncertainties suggested by Asquith (1998), the computed DDF and IDF values are considered applicable for the City of Austin in particular and Travis County in general. The DDF and IDF values are reported in tables 1 and 2, respectively. The IDF values were derived by simple division of DDF values by the precipitation duration in hours.

The durations include 15 minutes, 30 minutes, 1 hour, 2 hours, 3 hours, 6 hours, 12 hours, 1 day, 2 days, 3 days, 5 days, and 7 days. The frequency levels include the annual nonexceedance probabilities (F) of 0.5, 0.6, 0.7, 0.8, 0.9, 0.96, 0.98, 0.99, 0.996, and 0.998. These probabilities permit smooth graphical construction of the upper tail (F = 0.50) of the precipitation distribution. The recurrence intervals (RI) commonly used include 2, 5, 10, 25, 50, 100, 250, and 500 years. The F account for the RI. The 2-year RI corresponds to the median, 50th percentile, or 0.5 nonexceedance probability. The relation between recurrence interval and nonexceedance probability is RI=1/(1-F).

The distribution parameter values used in DDF computation provided by maps in Asquith (1998) were derived by computer algorithm using the latitude (30°17'42") and longitude (97°47'12") of Tom Miller Dam. The algorithm accuracy was manually verified. Small changes in the distribution parameters for the 7-day duration were made to insure that 7-day precipitation depths for the largest nonexceedance probabilities (F = 0.90) were larger than corresponding values for the 5-day duration. Upper tail convergence of the 5- and 7-day distributions appears likely.

Asquith (1998) defines separate durations of 60 minutes and 1 hour, which were derived from 15-minute data and hourly data, respectively, and separate durations of 24 hours and 1 day, which were derived from hourly and daily data, respectively. It was decided that the 60-minute and the 24-hour durations would not be used to define the DDF and IDF. This decision was based on the available record length and spatial distribution of 15-minute and hourly precipitation stations in the region surrounding the Austin area. Furthermore, a change in the distribution type from the Generalized Logistic to the Generalized Extreme Value occurs at the transition of 24 hours to 1 day.

REFERENCE

Asquith, W.H., 1998, Depth-duration frequency of precipitation for Texas: U.S. Geological Survey Water-Resources Investigations Report 98-4044, 107 p.

Table 1. Depth-duration frequency of precipitation for Austin and Travis County, Texas

[Note: Values in table derived from Asquith (1998). The location used to define the parameters of the precipitation distribution was Tom Miller Dam on the Colorado River near the center of the Austin area located at latitude 30°17′39″ and longitude 97°47′12″. min., minutes; hr, hours; in., inches]

Annual non-exceedance probability (percent)	Recurrence interval (years)	Precipitation depth and duration
		15 min (in)	30 min (in)	1 hr (in)	2 hr (in)	3 hr (in)	6 hr (in)	12 hr (in)	1 day (in)	2 day (in)	3 day (in)	5 day (in)	7 day (in)
0.500	2	0.98	1.32	1.72	2.16	2.32	2.67	3.06	3.44	3.81	4.04	4.30	4.57
.600	2.5	1.05	1.42	1.86	2.35	2.53	2.91	3.33	3.84	4.28	4.51	4.81	5.08
.700	3.33	1.14	1.54	2.04	2.58	2.79	3.19	3.64	4.33	4.84	5.08	5.43	5.70
.800	5	1.26	1.71	2.28	2.89	3.13	3.56	4.07	4.99	5.60	5.85	6.26	6.53
.900	10	1.47	1.98	2.68	3.42	3.71	4.21	4.81	6.10	6.88	7.14	7.65	7.91
.960	25	1.76	2.36	3.28	4.20	4.55	5.14	5.90	7.64	8.63	8.91	9.53	9.75
.980	50	2.01	2.68	3.79	4.88	5.28	5.94	6.86	8.87	10.0	10.3	11.0	11.2
.990	100	2.29	3.04	4.37	5.66	6.11	6.85	7.96	10.2	11.5	11.8	12.6	12.7
.996	250	2.73	3.57	5.26	6.86	7.38	8.24	9.67	12.0	13.6	13.9	14.7	14.8
.998	500	3.11	4.02	6.06	7.94	8.51	9.47	11.2	13.5	15.2	15.6	16.5	16.5

 

Table 2. Intensity-duration frequency of precipitation for Austin and Travis County, Texas

[Note: Values in table derived from Asquith (1998). The location used to define the parameters of the precipitation distribution was Tom Miller Dam on the Colorado River near the center of the Austin area located at latitude 30°17′39″ and longitude 97°47′12″. min., minutes; hr, hours; in./hr, inches per hour]

Annual non-exceedance probability (percent)	Recurrence interval (years)	Precipitation depth and duration
		15 min (in/hr)	30 min (in/hr)	1 hr (in/hr)	2 hr (in/hr)	3 hr (in/hr)	6 hr (in/hr)	12 hr (in/hr)	1 day (in/hr)	2 day (in/hr)	3 day (in/hr)	5 day (in/hr)	7 day (in/hr)
0.500	2	3.92	2.64	1.72	1.08	0.773	0.445	0.255	0.143	0.0794	0.0561	0.0358	0.0272
.600	2.5	4.20	2.84	1.86	1.18	.843	.485	.278	.160	.0892	.0626	.0401	.0302
.700	3.33	4.56	3.08	2.04	1.29	.930	.532	.303	.180	.101	.0706	.0452	.0339
.800	5	5.04	3.42	2.28	1.45	1.04	.593	.339	.208	.117	.0812	.0522	.0389
.900	10	5.88	3.96	2.68	1.71	1.24	.702	.401	.254	.143	.0992	.0638	.0472
.960	25	7.04	4.72	3.28	2.10	1.52	.857	.492	.318	.180	.124	.0794	.0582
.980	50	8.04	5.36	3.79	2.44	1.76	.990	.572	.370	.209	.143	.0917	.0668
.990	100	9.16	6.08	4.37	2.83	2.04	1.14	.663	.424	.239	.164	.105	.0758
.996	250	10.9	7.14	5.26	3.43	2.46	1.37	.806	.501	.283	.193	.123	.0882
.998	500	12.4	8.04	6.06	3.97	2.84	1.58	.934	.564	.318	.216	.137	.0982