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<br />Considerable uncertainty exists regarding the adequacy of simulator data <br />for determination of parameters for use in rainfall-runoff modeling. Apparent <br />differences between hydraulic conductivity values developed from the simulator <br />and those from rainstorms for some plots, and not for others, is not explained <br />easily. The scarcity of data from observed rainfall-runoff events, and the <br />difficulties encountered in collecting good data provide a poor foundation on <br />which to base final conclusions; additional data on observed rainfall-runoff <br />events on simulator plots are needed. <br /> <br />The application of PRMS to three storms on North Fork Willow Gulch in <br />1982, using estimates of KSAT obtained from rainfall-simulator runs on plots, <br />produced predicted runoff volumes that were about 70 percent less than those <br />observed in two cases, and 40 percent more than that observed in the third <br />case. Estimates of KSAT obtained from observed rainfall-runoff events on <br />plots improved the prediction for two events, and degraded the result for <br />the third. Adjustments in the KSAT specfication and adjustments to storm <br />rainfall produced close agreement between predicted and observed runoff <br />volumes, peak flow rates, and hydrograph shape. This exercise in parameter <br />adjustment only confirms the fact that accuracy of predicting peak-flow rates <br />is dominantly controlled by the amount of water that falls on the ground, and <br />how much infiltrates. Runoff routing is represented adequately by the con- <br />ceptual network of plane and channel segments used in PRMS. The problem of <br />the amount of water that falls on the ground is logistic and economic; it <br />could be resolved with more recording rain gages. The problem of the amount <br />of water that infiltrates can only be solved by a better understanding of <br />the onsite infiltration process and by the development of better models <br />based on this understanding. <br /> <br />58 <br />