The 10-year RKLS of the site may be determined as 

 follows: 



R = 31 X 8 = 248 

 K = 0.50 



LS = 1 



RKLS = 248 X 0.50 X 1.0 = 124 



124 > 74; therefore this amount of litter will not 

 protect the site if the litter is the only protection the site 

 has. If in addition to the 1 ton per acre (2.25 t/ha) of litter, 

 there is a 10 percent ground cover of grass and a 50 

 percent canopy of tall weeds, this would make the VM 

 factor equal to 0.19, figure 11. The site RKLS would 

 reduce VM to 0.19 X 124 = 23.6, thus fully protecting 

 the site from the 10-year R. 



R values have other uses. For example, suppose we 

 needed the 10-year runoff peak from a 1 -square-mile 

 watershed whose difference in elevation between the 

 top and bottom of the watershed was 1,000 ft (305 m) 

 and the R value at the watershed center was 30, the 

 equation developed by Fletcher and others (1977) 

 could be used as follows: 



- = 1 .2801 5 A - 56172 R094356 (DH) 0.16887 



The desired 10-year peak flow would then be 



q 10 = 1.28015 (1)0-56172 ( 3 0) 0.94356 (1 00) 01 6887 

 = 101.8 ft 3 /S (2886 liters/sec). 



Storm Occurrence by Storm Duration 



The percentage of short durations is appreciably 

 higher on the Straight Canyon zones than either at the 

 Davis County experimental watersheds or the Ephraim 

 experimental area. Figure 1 1 shows the cumulative 

 frequency distribution of storm durations. The correla- 

 tion between elevation and duration can be seen in the 

 mean values for each zone where zone 1 mean eleva- 

 tion 9,045 ft (2 757 m) has 33 percent of the storms 

 shorter than 36 minutes, zone 2 mean elevation 1 0,065 

 ft (3 068 m)has22 percentof the storms shorter than 36 

 minutes, and zone 3 elevation 7,765 ft (2 367 m) has 41 

 percent of the storms shorterthan 36 minutes. Inciden- 

 tally, only one storm at one location lasted up to 15 

 hours. 



STORM DURATION The duration distribution of the 

 1,228 storm events on the Straight Canyon barometer 

 watershed are shown in figure 1 2. The longest duration 

 recorded for any storm was 1 5 hours, with 90 percent of 

 the storms having durations shorter than 5 hours and 

 approximately half of the storms having durations 

 shorter than 1 hour. 



STORM DEPTHS The largest single storm depth 

 was 1.50 in (38 mm). Figure 13 shows the frequency 

 districution of the 1,228 storm depths. Note that more 

 than 65 percent of the storms have depths smaller than 

 0.15 in (3.8 mm). The depth distribution of storms at 

 Straight Canyon essentially fits a log normal distribu- 

 tion as evidenced by the straight line in figure 13. 



o 



I— 



< 



ACCUMULATIVE PERCENTAGE OF STORMS 



Figure 12 — The frequency distribution of durations of 1,228 

 storms on the Straight Canyon barometer watershed, Utah. 



1.0 



o 



0. 1 



i mi 1 1 i i ii n 



5 20 40 60 80 90 95 98 99 99. 9 



ACCUMULATIVE PERCENTAGE OF STORMS 



Figure 13. — The frequency distribution of storm depths for 1,228 

 storms on Straight Canyon barometer watershed, Utah. 



9 



