The precipitation values used in making the zonal curves contained some snowfall 



events. At the higher elevations, snow has been recorded during every month of the 



year. However, there is little doubt that the values for periods of 2 hours and shorter 

 for recurrence intervals of 10 years and longer are rainfall events. 



Close examination of figures 3 and 4 reveals that generalized comparisons are dif- 

 ficult to make. Comparisons should be confined to specific durations and recurrence 

 intervals because the magnitude of the 2-year intensity and the slope of the curves 

 vary vs^ithin and between zones. For instance, on the Davis County area the expected 20- 

 minute, 50-year intensity is greatest in Zone 1, while the 20-minute, 2-year intensity 

 is greatest in Zone 3. 



Zone 2, aspen-fir t>'pe, in the Davis County area occupies the bottom of the higher 

 reaches of Farmington Canyon. All of this zone lies behind a topographic barrier with 

 respect to the prevailing southwest storm direction, and is the only zone on the leeward 

 slope. The physiographic effect in Zone 2 was not determined precisely; however, com- 

 pared to Zones 1 and 3, it appears that for durations less than 2 hours the expected 

 intensities decrease--especially at long recurrence intervals. 



On both study areas, as elevation increases, there is a trend toward reduced 

 intensities at the longer recurrence intervals. However, this trend is not uniform. 

 On each area maximum expected intensities are usually found in the precipitation zone 

 occupied by the oakbrush type. Fortuitously, this vegetal type also offers good protec- 

 tion against raindrop impact and overland flow. 



Expected rainfall intensities in the aspen-fir type on both study areas are some- 

 what less than those in the oakbrush type at most durations and recurrence intervals, 

 except at the short durations with short recurrence intervals. This exception is 

 probably due to the increased frequency of storm occurrence with increasing elevation. 

 The intensity-duration-frequency curves representative of the higher spruce-fir zone 

 are more steeply inclined than those of the aspen-fir type. In both zones the expected 

 rainfall intensities at short recurrence intervals are about the same, but they are 

 usually greater at the long recurrence intervals in the spruce-fir t>^e. However, the 

 expected intensities in this type are usually less than those in the oakbrush type. On 

 the Davis County Experimental Watershed, Zone 4 in the spruce-fir t>'pe is not markedly 

 different from Zone 3, also in the spruce-fir ty^e , except that the 2-minute duration 

 values differ only slightly from the 5-minute values. 



24 -Hour Depths 



Only a small percentage of storms that occur on either study area have durations 

 greater than 6 hours. However, these storms are of interest in the design of contour 

 trenches and road-drainage devices. Storms of long duration are rarely flood-producing 

 events in these areas. Croft and Marston (1950) indicate that the greatest storms on 

 the Davis County Experimental Watershed averaged much less than 6 hours; they ranged 

 from 30 minutes to 10-1/2 hours. 



The intensities were converted to depths because depth is of more interest than 

 intensity at the 24-hour duration. The recurrence intervals shouTi in table 3 are the 

 average intervals in which the given depth will recur as an annual seasonal maximum. 



Monthly Precipitation Depth and Number of Storms 



The monthly precipitation depths were determined from gage catch. We hoped that 

 these data could clarify the mountain-valley precipitation relationships when used in 

 conjunction with intensity-duration-frequency characteristics. 



14 



