DISCUSSION 



In general, the mountain-valley intensity relation- 

 ships stated by Brancato (1 942) hold truefor these data. 

 He stated: 



With regard to variation of thunderstorm 

 rainfall with elevation . . . over a long period of 

 time a station located at a lower elevation is 

 likely to experience the most intense thun- 

 derstorm. 



Dorroh (1946) also substantiates this hypothesis. 



Plant cover destruction resulting in active flood and 

 sediment source areas has occurred prevalently on 

 high-elevation herbaceous sites that lie above the 

 aspen-fir type, and was due primarily to summergrazing 

 overuse by livestock. Our data show that the rainfall 

 intensities expected to occur on such sites are quite 

 substantial. 



Some of the rainfall intensities that can be expected 

 to occur probably will be greater than the infiltration 

 capacities of some sites, particularly those in poor 

 hydrologic condition. Hence, overland flow is almost a 

 certainty. Fortunately, management practices on moun- 

 tain watersheds can drastically alter runoff volumes, 

 flood peaks, and erosion. This has been amply and con- 

 vincingly demonstrated on the Davis County experi- 

 mental watershed (Bailey and others 1 934; Bailey and 

 others 1 947). On both study areas in the middle 1 930's, 

 severe mudrock floods were generated by storm events 

 with a recurrence interval of only 15 years. Since that 

 time, both vegetal and mechanical rehabilitation mea- 

 sures have resulted in the satisfactory disposition of 

 storm rains of equal or greater magnitude. 



The greatest annual rainfall intensities can be ex- 

 pected at the lowest elevation on the Straight Canyon 

 barometer watershed. This is different than on the 

 Ephraim side of the mountain but similar values were 

 obtained at Oaks climate station, which is the same 

 elevation band as zone 3 on Straight Canyon. 



Depth-duration curves suggest that the longer the 

 storm, the greater the runoff. Osborn (1 964) has pointed 

 out that the use of depth-duration data can result in 

 misleading runoff values. He reported that, in the semi- 

 arid Southwest rangeland, major runoff events are often 

 the result of short-lived, high-intensity convective 

 storms. Osborn's conclusion is generally applicable to 

 our study areas. Major amounts of summer runoff will 

 usually come from storms of medium duration, namely 2 

 to 6 hours, with short periods of high-intensity rainfall 

 bursts. 



The slope of the depth duration curves of Straight 

 Canyon all lie within the range of values found on the 

 Great Basin experimental area. 



Elevation significantly affects 10-minute, 10-year 

 precipitation intensity at Straight Canyon and Great 

 Basin but not on the Davis County area. Precipitation 

 intensity decreases more than an inch per hour as 

 elevation increases 5,000 ft (1 524 m). 



A better relationship exists between the 10-minute, 

 1 0-year precipitation and the miles of penetration from 

 the Wasatch-Pavant front. This relationship has a coef- 

 ficient of variation of about 60 percent on the Straight 

 Canyon area, decreasing to 50 percent on the Great 



Basin area and becoming nonsignificant on the Davis 

 County area. 



Although the mountain-valley intensity relationships 

 tended to follow Brancato's (1 942) thesis that the lower 

 lying stations receive the most intense rainfall, our data 

 do not support his contention concerning the amount of 

 rainfall. He stated: 



Three to four times as many thunderstorms 

 occurred on the middle and upper windward 

 slopes of the mountains as on the relatively 

 flat and lower portions of the basin. However, 

 contrary to published and popular accounts, 

 the thunderstorms produced the greatest 

 amount of precipitation at the lower eleva- 

 tions and not on the mountain slopes. The 

 most favorable conditions for the production 

 of heavy rain is the presence of an air mass 

 with a sufficient amount of available energy 

 and the greatest possible amount of mois- 

 ture. Orographic lifting is very effective as a 

 mechanism to release the latent energy in an 

 air mass, but as the air is lifted over progres- 

 sively higher terrain, the total amount of 

 available precipitable water above any given 

 area becomes progressively smaller. 

 Two assumptions upon which Brancato bases his 

 thesis might be questioned. One is the assumption that 

 the amount of precipitable water in an airmass becomes 

 significantly less as it is forced over a single mountain 

 crest. It is not likely that the total precipitable water 

 changes very much on adjacent precipitation zones. 

 Significant diminution of precipitable water requires the 

 passage of an airmass over substantial sections of 

 terrain. Another questionable assumption is that oro- 

 graphic lifting is the dominant mechanism triggering 

 storms. Orographic effects may well contribute to 

 summer shower activity, but the local daytime slope 

 heating and induced upslope breezes are probably 

 more important. Cold fronts and uppertroughs may also 

 contribute to some of the storms, especially early and 

 late in the season. These may act in conjunction with 

 daytime surface heating and orographic lifting. 



The relation between elevation and number of 

 storms is similar to that between elevation and average 

 monthly rainfall depth. The ratio between the number of 

 storm occurrences at the highest zones and at the 

 lowest zones varies between 1.0 and 1.2. 



The Straight Canyon 60-minute rainfall intensities 

 are more like those given by Farmerand Fletcher(1 971 ) 

 for southern Utah than central Utah. Other observa- 

 tions regarding area of application are similar. 



On the basis of the zonal R values, both erosion and 

 runoff peaks should be about 1 .3 times as great in zone 

 3 as in zone 1. 



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