Frequency Analysis 



A detailed annual series frequency analysis of rainfall intensity was made for every 

 station. A separate analysis was made for each of the 12 time durations. The formula 

 developed by Weibull was used to obtain plotting positions (Chow 1964) . 



T = 



where 



n+1 

 m 



T = recurrence interval, years 

 n = number of years of record 



m = order number of the items arranged in descending order. 



This formula has been found to be theoretically suitable for plotting annual maximum 

 series on extremal distribution paper (Chow 1953) . However, the data in this study were 

 more nearly straight on extremal log paper. 



Precipitation Zones 



Peck and Brown (1962) divided Utah into 20 precipitation regions. They found that 

 a large amount of the variation between regions in the May-September precipitation was 

 accounted for by elevation. All of our data are point data. Consequently, we had to 

 define criteria for dividing the study areas into homogeneous zones in order to make 

 areal application of these point data. 



The three criteria used follow: (1) station elevation; (2) the values from the 

 station intensity-duration-frequency curve; and (3) the station I10/I2 ^atio for all 

 durations between 2 and 30 minutes (figs. 1 and 2, tables 1 and 2). The latter is a 

 dimensionless ratio that expresses the average slope of the short-duration rainfall 

 intensity curves between 2 and 10 years. This ratio was computed by dividing the summa- 

 tion of intensities having durations of 2, 5, 10, 15, 20, and 30 minutes for the 10-year 

 recurrence interval by the comparable stmmiation for a recurrence interval of 2 years. 

 Any region that is homogeneous with respect to its rainstorm characteristics should have 

 frequency curves of about equal slope or steepness. Consequently, the I10/I2 ratio is 

 a useful statistic for comparing the slopes of frequency curves as a basis for judging 

 homogeneity of precipitation zones. 



24 - Hour Precipitation Depth 



The intensities for the 24-hour duration were converted to precipitation depths for 

 recurrence intervals of 10, 25, and 50 years. Twenty-four hours was the longest duration 

 examined in this study. 



Average Montlily Dept/i and Number of Storms 



Analyses for the average monthly depth of precipitation and the average number of 

 storms per month are complete only for June-September. At the highest elevation stations, 

 the data for May and October were insufficient to compute a reliable monthly average. 



Storm Occurrence by Hour 



Storm occurrence by hour was analyzed by compiling the number of storms starting in 

 any hour of the day expressed as a percent of the total storms. These data were plotted 

 as a mass curve for each zone. 



6 



