Storm Duration 



The duration of all storms at all precipitation gages 

 was divided into 0.25-hour units and the frequency in 

 each group recorded and expressed as a percentage of 

 the total number of storm events. These percentages 

 were plotted against the unit of duration in which each 

 occurred. 



RESULTS 



Consistency Test 



The records were not adjusted. None of the mass 

 curves plotted as smooth straight lines and isolated 

 points fluctuated both above and below the trend line. 

 Even so, the breaks in the lines didn't persist for a period 

 as long as 5 years. The breaks in the lines were 

 considered to be no greater than might reasonably be 

 expected for thunderstorm data obtained from moun- 

 tainous areas. Furthermore, none of the differences 

 between the first and second half of the records were 

 significant so the records could be said to be consistent. 



Precipitation Zones 



Within the study area the precipitation zones are 

 related to the vegetal patterns (table 2), as was noted by 

 Farmer and Fletcher (1 971 ), even though the contrasts 

 in vegetal types are not as great at Straight Canyon as 

 on the Davis County or Ephraim watersheds. 



Intensity-Duration-Frequency Characteristics 



The curves in figures 2, 3, and 4 are for recurrence 

 intervals of 2 to 50 years and storm durations between 2 

 minutes and 24 hours. All return period intensity values 

 for 1 years and longer were determined by linear inter- 

 polation. As such, these must be used with caution. Also, 

 the recurrence interval is the average interval during 

 which an intensity of a given duration will recur as an 

 annual seasonal maximum. 



Snow may have occurred during any month of the 

 year at the higher elevations, and thus was probably 

 caught in all of the higherelevation gages. There is little 

 doubt, however, that values for periods shorter than 2 

 hours and recurrence intervals longerthan 10yearsare 

 from rainfall events. 



6 r 



RECURRENCE INTERVAL (YRS) 



Figure 2— Rainfall intensity-duration- 

 frequency curves for Straight Canyon ba- 

 rometer watershed, Utah, zone 1. 



The rainfall intensities in each precipitation zone are 

 appreciably different from those in any other zone. For 

 example, for a duration of 2 minutes and 2-year recur- 

 rence, the intensity is 1 .20 in (30 mm)/h for zone 1 , 0.30 

 in (8 mm)/h for zone 2, and 1.65 in (4.2 mm)/h for zone 3. 

 At a 50-year recurrence, the 2-minute intensities be- 

 come 4.52 in (1 1 8 mm), 7.65 in (1 94 mm), and 1 3.50 in 

 (343 mm)/h, respectively. The 50-year/2-year ratios for 

 a 2-minute duration become 3.77 in (96 mm), 25.5 in 

 (648 mm), and 8.1 8 in (208 mm)/h for zones 1 , 2, and 3, 

 respectively. 



2 5 10 25 50 



RECURRENCE INTERVAL <YRS) 



Figure 3 —Rainfall intensity-duration- 

 frequency curves for Straight Canyon ba- 

 rometer watershed, Utah, zone 2. 



2 5 10 25 50 



RECURRENCE INTERVAL (YRS) 



Figure 4— Rainfall intensity-duration- 

 frequency relationship curves for Straight 

 Canyon barometer watershed, Utah, zone 

 3. 



4 



