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<br />time between beginning of rainfall and recording of runoff as an indication of <br />uniform precipitation over the basin, and (4) the preference for larger rainfall- <br />runoff events during which the entire watershed responds. The range in the <br />peak discharges of the floods used for model calibration is shown for each station <br />in figure 2. These 141 floods ranged in peak discharge from less than 10 ft3/S <br />to 3,300 ft3/S, but only 20 floods occurring at just 10 of the sites had unit <br />discharges exceeding 100 {ft3/s)/mj2. <br /> <br />For the 141 storms, the average and maximum total rainfall amounts recorded <br />at 5-minute intervals is shown in figure 3. Greatest average total precipitation <br />was 0.16 inch (1.9 in. /hr) occurring in the interval between 5 and 10 minutes <br />after the storm began, and the "average storm" lasted about 45 minutes. The <br />maximum rainfall accumulation recorded in a 5-minute time period was 0.51 inch <br />(6.1 in./hr). <br /> <br />The seasonal occurrence of the 141 floods is shown by 10-day intervals in <br />figure 4 for the approximate period May to September. About 78 percent of these <br />floods occurred between May 21 and August 18 (about 28 percent occurred be- <br />tween July 20 and August 8). In a similar analysis of flood occurrences in the <br />Front Range Urban Corridor of Colorado, Hansen, Chronic, and Matelock (1978) <br />found 83 percent occurred between May 21 and August 18. <br /> <br />The rainfall-runoff simulation model requires daily precipitation and daily <br />evaporation in addition to unit (5-min) storm rainfall and runoff data. Daily <br />precipitation was as recorded either at the site or at the nearest U. S. Weather <br />Service station during missing record periods. Dai Iy evaporation was from the <br />closest of either Pueblo City Reservoir Qr John Martin Reservoir, National Weather <br />Service stations 6743 and 4388, respectively. For the period of seasonal station <br />operation, these data and the observed unit storm data are used to calibrate the <br />infiltration, soil-moisture accounting, and surface-runoff-routing components in <br />the model. <br /> <br />Calibration of Rainfall-Runoff Model <br /> <br />The calibration phase of the modeling process resulted in an optimum set of <br />10 parameters for each basin. An explanation of each parameter and its appl ica- <br />tion in the modeling process is modified from Litchy and Liscum (1978, p. 35) in <br />the following tabulation: <br /> <br />10 <br />