Depth-Duration Frequency of Precipitation Maps for Texas CE 394K Meghan Strand

Outline Background Information Procedure Results Analysis of Results GLO and GEV Distributions Parameter Maps Procedure Results Analysis of Results

Background Information Depth-Duration Frequency of Precipitation for Texas, Water-Resources Investigations Report 98-4044, published by the USGS in 1998 by William Asquith Report provided procedures for attaining precipitation depth for a given frequency using specific distribution equations. Report provided probability distribution parameter maps (location, scale, and shape) derived from L-moment statistics of annual precipitation maxima. Research based on data from 173 fifteen-minute, 274 hourly, and 865 daily NWS Precipitation Stations. Term Project expands on the developments of the 98-4044 report - its purpose is to create depth duration frequency maps from the existing parameter information.

Generalized Logistic and Generalized Extreme-Value Distributions Generalized Logistic (GLO) Distribution Selected for durations of 15 minutes to 24 hours. Xd(F) = Depth of precipitation. F = Annual nonexceedance probability ,or 1-(1/T) where T is the recurrance interval. Generalized Extreme-Value (GEV) Distribution Selected for durations of 1-day and greater. ln(F)=natural logarithm of annual nonexceedance probability. ξ,α, and κ = location (position on number line), scale(tilt of curve), and shape (shape of curve) parameters for both distributions.

Parameter Maps Location (ξ ) parameter of generalized logistic (GLO) distribution for 3-hour precipitation duration in Texas. These parameter maps can be combined in the distribution equations to create depth duration frequency maps.

Procedure For each parameter a text file existed which the author converted to rasters using the ASCII grid command. A shape file of Texas counties was also imported and the projection modified to Clarke 1866 Albers Equal Area. The grids were then combined using the raster calculator and AML language with the equations shown here: Note: The initial OUT1 depends on duration which determines which distribution equation. κ is a grid until the 1-day duration where it becomes a constant because the statewide mean was very similar to the determined value and could be used for 1- to 7-day durations.

Precipitation Depth for a 3-Hour Duration, Results Precipitation Depth for a 3-Hour Duration, Maps were created for 14 durations (15, 30, and 60 minutes; 1,2,3,6,12, and 24 hours; and 1,2,3,5, and 7 days. Maps were created for 2, 10,25,50,100,250, and 500 year storm frequencies. A total of 96 maps were constructed. 100-Year Storm Event

Analysis Newton County is increasing monotonically while El Paso County is showing some variation. Inconsistencies in frequency curves will have to be analyzed.

Analysis of Inconsistencies There should be no negative values when a storm of lesser duration is subtracted from one of greater duration for the same storm frequency. Inconsistencies are attributed to: differing number of data values for each duration Change in distributional form at the 24 hour to 1 day duration. Incomplete interpolations of data contours. Future analysis is necessary to produce consistent depth-duration frequency maps.

William Asquith (USGS) Special Thanks William Asquith (USGS) Natalie Houston (USGS) David Maidment, PhD (UT-Austin)

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