The summer precipitation burst is considered to be a 

 reflection of the summer Gulf of California monsoonal 

 storms as well as orographic convection. The monsoonal 

 storms generally peak during August but may occur in the 

 period June through October. 



Storm Occurrence by Hour 



On the average, storms occur on the Straight Canyon 

 barometer watershed most frequently between 1100 

 and 1300 hours. Figure 6 shows the percentage of 

 storms occurring during each period of the 24-hour day. 

 Note that few storms occur between 2100 and 900 

 hours. This observation agrees with the distributions 

 found on the Great Basin experimental area (Farmer 

 and Fletcher 1971). Both these curves differ signifi- 

 cantly from the curves for Davis County. 



The concentration of the storms in the afternoon is 

 expected because convection must trigger the mon- 

 soonal storms as well as orographic convective storms. 



Storm Penetration 



The storm penetration distances in miles downwind 

 from the Wasatch-Pavant fronts for each gage on the 

 Straight Canyon barometer watershed are shown in 

 table 2. A good, simple correlation with a log-log trans- 

 form exists between the miles of penetration and 10- 

 year, 10-minute precipitation intensity. The correlation 

 coefficient, r 2 = 0.59, is significant at the 1 percent 

 probability. When all gages are included in the relation- 

 ship, the gages on the windward slopes dominate the 

 relationship and the correlation reverses to become 

 significantly negative at the 5 percent probability level. 



Within Straight Canyon the 1 0-year, 1 0-minute rain- 



L4 ,- 



A 



f | Zone 2 



I Elevation 10, 065 ft 

 1.2 - 1 



1 

 I 



Elevation 8, 890 tt y\ \ 



JAN. FEB. MAR. APR. MAY JUNE JULY AUG. SEPT. OCT. NOV. 



MONTH 



Figure 5— The mean monthly precipitation 

 depths for Straight Canyon barometer wa- 

 tershed zones 1, 2, and 3 and Castle Dale, 

 Utah. 



fall intensity increases from 1.55 in (39 mm) to 2.85 in 

 (72 mm) per houras penetration increasesfrom 20to30 

 miles. On the other hand when all three locations are in 

 the regression, the 10-year, 10-minute precipitation 

 intensity decreases from 3.00 in (76 mm) per hour at 0.7 

 miles to 2.30 in (58 mm) per hour at 30 miles. 



Erodent Values, R 



The mean annual El values for the three locations 

 may be seen in tables 2 and 5 for each of the precipi- 

 tation zones. Since R values are determined from log 

 probability plots of the annual El values, it isimportantto 

 know not only the R ormean annual (2-year) El value but 

 the slope of the line that was used in the rainfall 

 frequency depth curves. The ratio of the 10-year El 

 value to the 2-year (R10/R2) gives this slope. Then an El 

 value for any frequency greater than the mean annual 

 value (R 2 ) can be determined. 



For periods of timeshorterthan 1 yearthecurvesasa 

 percentage of the mean annual R value are of use. 

 Figure 7 shows the monthly percentage of the annual R 

 value that occurs from the beginning of the season to 

 each date to the end of the season for each of the three 

 precipitation zones of the Straight Canyon barometer 

 watershed. Figure 8 shows the same data for the Great 

 Basin experimental area zones and figure 9 shows the 

 same data for the Davis County experimental water- 

 shed zones. 



Use of R Values 



Wischmeier and Smith (1958) presented a method 

 for utilizing the R value as a parameter for estimating 

 mean annual erosion east of the Rocky Mountains. Utah 

 State University and Intermountain Forest and Range 

 Experiment Station (1976) extended the procedure to 



TIME OF DAY 



Figure 6— The frequency distribution of 

 storms by hour of the day at Straight Can- 

 yon barometer watershed, Utah. 



6 



