Precipitation Zones 



Peck and Brown (1 962) divided Utah into 20 precipi- 

 tation regions. They found that a large amount of the 

 variation between regions in the May-September preci- 

 pitation 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 were: (1 ) station elevation; (2) 

 the valuesfrom the station intensity-duration-frequency 

 curve; and (3) the station I10/I2 ra tio for all durations 

 between 2 and 30 minutes (fig. 1, tables 1 and 2). The 

 latter is a dimensionless ratio that expresses the aver- 

 age slope of the short-duration rainfall intensity curves 

 between 2 and 10 years. This ratio was computed by 

 dividing the summation of intensities having durations 

 of 2, 5, 10, 15, 20, and 30 minutes for the 10-year 

 recurrence interval by the comparable summation for a 

 recurrence interval of 2 years. Any region that is 

 homogeneous with respect to its rainstorm characteris- 

 tics should have frequency curves of about equal slope 

 or steepness. Consequently, the I10/I2 ratio is useful 

 statistic for comparing the slopes of frequency curves 

 as a basis for judging homogeneity of precipitation 

 zones. 



Average Monthly Depth and Number of Storms 



Analyses for the average monthly depth of precipita- 

 tion and the average number of storms per month are 

 complete only for June-September. At the highest ele- 

 vation 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 com- 

 piling 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. 



Storm Penetration 



Storm penetration is a measure of the distance a 

 storm travels downwind from the first uplift barrier, in 

 this case the Wasatch and Pavant Mountain Ranges. 

 The greater the penetration, the less precipitation falls 

 at a given elevation. 



Erodent, R, Values 



The erodent values for each precipitation station 

 were calculated from the unit intensity and volume by 

 the relationship: 



24-Hour Precipitation Depth 



The intensities for the 24-hour duration were con- 

 verted to precipitation depthsfor recurrence intervals of 

 1 0, 25, and 50 years. Twenty-four hours was the longest 

 duration examined in this study. 



O 



where 

 El 



I 



'30 



R 



East Mtn. 



Ranger Station 



Danish Knoll 



Can 



N 



yo n 



6 + 331 log I 



100 



X P 



) 



x I 



30 



= rainfall erosivity index for 1 year or other 

 period 



= rainfall intensity for a short unit of time 

 = volume of precipitation in the same unit of 

 time 



= the annual maximum, or other period as 

 for El, 30-minute rainfall intensity 



= the mean annual El value as derived from 

 a frequency plot of annual El values. (See 

 Wischmeier and Smith 1958.) 



flORANGEVILLE 



Figure 1—Map of Straight Canyon barometer watershed Utah, showing zones and gages. 



3 



