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accumula~ions under about 0.20 inch made a relatively minor contribution. Based on these comparisons the same conclusions can be reached for the storm periods affecting the Arizona high plateau region. 3.2.6 Diurnal Variation of Precipitation The diurnal variation of hourly precipitation is displayed in Figure 3.3. Because of the relatively small data base (total hours about 1000 with precipitation equal to or greater than 0.01 inch from the seven gauge network and total precipitation about 54 inches), the data were combined into three hour averages beginning at midnight to 0300 MST, 0300-0600 MST, etc. This gave eight relatively smooth data points over the 24 hour period. The frequency 'of occurrence of precipitation (bottom curve, solid line) for any of the three hour segments varied from a low of 7.2 percent of all the hours that had precipitation equal to or greater than 0.01 inch, to a high of 11.6 percent. The low came at the three hour segment ending at 0900 MST and the high came three hours later (ending at 1200 MST). The general trend was for precipitation to occur more frequently from midmorning until late evening (about 2100 MST) with a slight decrease in frequency of occurrence thereafter, until midmorning. However, there was also some tendency for a decrease in frequency of occurrence during the midafternoon period. The percentage of total precipitation (top curve, dashed line) plot for the three hourly averages revealed the greatest amount of precipitation fell during the daylight hours, peaking near noon, with 17.3 percent of the precipitation falling during the 0900-1200 MST period. The least precipitation 3-37 I . I i I I I I I I I I I I I I I I I