Table S.--The ranges of 60-minute rainfall intensities ^ in 

 inches per hour^ for the 10- and 50-year 

 recurrence intervals by regions 



Region 10 years \ 50 years 



Central fi southern Idaho 

 Northern Utah 

 Central Utah 

 Southern Utah & 



northern Arizona 

 Southern Arizona 



0.22-0.45 

 0.37-0.86 

 0.62-0.86 



0.38-0.77 

 0.96-2.20 

 0.98-1.20 



1.11-1.50 

 1.61-1.99 



1.69-2.00 

 2.22-2.60 



What is the areal applicability of the intensity data? In addition to the informa- 

 tion presented here, data are available for 41 additional intensity gages located in 

 New Mexico, Arizona, Utah, and Idaho. The additional data include intensity- frequency 

 information for the 60-minute durations (table 5) . 



The trend toward greater intensities is inescapable if one examines the hourly 

 data at the most northern gages, which are in central Idaho, and then progressively 

 moves to the more southerly stations in southeastern Idaho; northern, central, and 

 southern Utah; and northern and southern Arizona. Some anomalies are present, but this 

 is expected since these stations are grouped irrespectively of elevation, physiography, 

 and source of summer moisture. For any given recurrence interval, our data (figs. 3,4) 

 probably underestimate the 60-rainute rainfall intensities for stations in southern 

 Utah and Arizona and overestimate those for southern and central Idaho. This rel tion- 

 ship is expected to hold for all durations from about 30 minutes to 24 hours. 



Evidence that rainfall intensities in all of these areas may be reasonably compar- 

 able for durations shorter than 30 minutes exists in maximum observed 2-minute intensi- 

 ties for selected stations in Idaho, Utah, and Arizona as follow: Reynolds Creek, 

 Idaho, 17.08 inches per hour; Oaks Climatic Station, Utah, 14.40 inches per hour; and 

 Tombstone, Arizona, 18.21 inches per hour. The maximum differences in these intensities 

 only amount to 0.13-inch depth in 2 minutes. 



Kidd (1964), working in south-central Idaho, reported the 15-minute, 4-year return 

 period intensity as 3 inches per hour. His period of record was only 5 years. This is 

 about an inch per hour greater than would be expected in northern and central Utah. 



Keppel and Fletcher (1958) reported the 20-minute, 100-year rainfall intensity as 

 6 inches per hour in southern Arizona. This is very close to the extrapolated 100-year 

 value of 5.8 inches per hour for precipitation Zone 1 on the Davis County Experimental 

 Watershed. They also reported that 80 percent of the storms lasted less than 4 hours. 

 This compares quite favorably with our findings. 



The transposition of these data to other areas should be approached cautiously. . 

 However, for low-risk hydrologic design problems, the data for durations less than 30 

 minutes are probably representative of many windward mountain slopes throughout southern 

 Idaho, eastern Nevada, Utah, and Arizona. At durations of 30 minutes and greater, rain- 

 fall intensities reported here are probably too high for the northern part of this re- 

 gion and too low for the southern part. 



22 



