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~ Five-minute rainfall data for six storms for two gages at Surrey Downs (fig~ IA) and five gages at Harvard Gulch (fig. IB) were provided. Information on antecedent conditions including the amount of precipitation In the past 1,3,7, and 14 days and the time since the last 0.0, 0.2, 0.5, and 1.0 in. of rain was provided for each storm at Surrey Downs. No antecedent conditions Were reported for Harvard Gulch because of the semiarid climate, but it was reported that the area is routinely irrigated with 1.0 to 1.5 inlwk which is considered equal to the rate of evaporation. Daily evaporation at the Surrey Downs watershed ranged from about 0.30 inches in the summer to about 0.02 inches during the winter. The rainfall and antecedent conditions for the storms sinJulated are summarized in table 2. Storms at the Harvard Gulch site averaged about 1.5 hours, and those at the Surrey Downs site averaged about 15 hours. Rainfall intensity in the Harvard Gulch watershed was generally 5 to 6 times greater on average than in the Surrey Downs watershed. Often, the 60-minute maxinJum intensity recorded in the Harvard Gulch watershed by a single raingage exceeded the average rainfall volume for all gages indicating a wide spatial variability. Rainfall in the Surrey Downs watershed was relatively uniform. COMPARISON OF SIMULATE}) AND OBSERVED FLOWS Results of the model-simulated flows and storm volumes (fig. 2 and 3) are summarized in terms of the' percent error (departure fro~erved values) in table 3 (peak flows) and table 4 (storm volumes). The percent errods computed as; r = (predicted-Observed). 100 (1) Observed where: predicted Is the simulated value offlow or volume. and observed Is the measured value of flow or volume CASC2D results were obtained only for Harvard Gulch for the May 8, 1981 storm; the results are not included in the tables, but are shown in figure 2. Error for CASC2D sinJulations was -10 percent for peak flow and -39 percent for storm volume. Also, re$ults ofCHUP/SWMM, the distributed version of the CUHP model linked using SWMM, is only available for the Harvard Gulch watershed because no sinJulations were made of the Surrey Downs watershed. Simulated peak flows differed from observed peak flows (table 3) by -100 to 260 percent at Harvard Gulch and by -100 to 200 percent at Surrey Downs. The RMS peak flow error ranged from 19 to 171 at Harvard Gulch and from 40 to 162 at Surrey Dowrus. The average RMS peak flow error for all models was 55 for Harvard Gulch and 49 fur S'h- rey Downs. The standard deviation of the error in the predicted peak flow ranged from 17 to 92 at Harvard Gulch and from 20 to 85 at Surrey Downs. The average standard deviation in peak flow model error is about 40 percent greater at Surrey Downs (40) then at Harvard Gulch (31). In general, sinJulated peak flows at the Surrey Downs site tend to be overpredicted, whereas sinJulated peak flows for some models are overpredicted and some are llnderpredicted at Harvard Gulch. PEAK FLOW 25 , 2.500 ., ~, 20r + / 2,000 ~ v / EXPLANATION - / ~ / 0 + <1 / . CASC2D ~- -z 1,000 / + CUHP ~o 10 'i' / <1/ OU X CUHP/SWMM ..JW .+ ~ / 700 <1,.t u.(J) DR3M -- "'0: 7~ Q ~ 500 . / ~ e 0 <w 5~ 3+ / !! 400 . . HEC Wo. r / ~- 0.>- 4 300 <1/ - 0 HSPF - Ow ~7 <1. /~ v ww 3 200 t>. PSRM >-u. /~ 06' '. ~Q v ~~ -1 & <l SWMM , ::JaJ 2 ::!::J / ... 6 100 11 .., TR20 (iiU / /lI- ~ / A. SURREY DOWNS 70 / !:!6 B. HARVARD GULC~ 1 . , 50 - 1 2 5 10 2025 50 100 200 500 1,000 2,500 OBSERVEO PEAK FLOW. IN CUBiC FEET PER SECOND Figure 2.-8imuiated versus observed peak flows at (A) Surrey Downs, WA and (B) Harvard Gulch, CO~ for six storms [CASC2D results only available for one storm at Harvard Gulch, Locations shown in fig 1.] 7-167 '... i: