Fortunately, the greatest annual rainfall intensities can be expected to fall on 

 the oakbrush type. Historically, flood source areas have not existed in this vegetal 

 type. However, vegetal conversion as a means of increasing water yields from the oak- 

 brush type has been considered. The potential hydrometeorological implications of such 

 conversions are clear--the resulting vegetal type should be able to handle storm rain- 

 fall of high intensity without producing serious flood concentrations or soil erosion. 

 Regardless, urban encroachment and the paving of areas within the oakbrush type can be 

 expected to produce localized flooding. 



Depth-duration curves suggest that the longer the storm, the greater the runoff. 

 Osborn (1964) has pointed out that the use of depth-duration data can result in mislead- 

 ing runoff values. He reported that, in the semiarid 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, i.e., 2 to 6 hours, with short periods 

 of high-intensity rainfall bursts. 



Apparently, no firm relationship exists between summer precipitation on the lowest 

 and on the highest zones. Clyde (1931) was unable to establish a relationship between 

 precipitation in the valleys with that in the mountains. The long term averages of 

 monthly depth show that the ratio between highest and lowest zones varies between 1.2 

 and 4.1. But in any given year, these ratios may be quite different. Nonetheless, there 

 is a definite trend toward greater average monthly depths with increasing elevation. 

 There are no indications that precipitation depth decreases above a certain altitude. 



Although the mountain-valley intensity relationships tended to follow Brancato's 

 (1942) 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 elevations and not on the mountain slopes. The most favorable 

 condition 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 moisture. Orographic lifting is very effective as a mechanism 

 to release the latent energy in an air mass, but as the air is lifted 

 over progressively 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 is the assumption that orographic 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 upper troughs 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 0.7 and 1.6. 



21 



