A previously developed water balance model was modified to predict the rate of surface runoff and erosion from small agricultural watersheds continuously over the growing season and on an individual storm basis. The modified model included: (1) A subroutine to calculate potential evapotranspiration using pan evaporation data. (2) Modification of infiltration subroutine to consider the effect of tillage and rainfall kinetic energy on infiltration capacity. (3) An overland flow routing component to route the excess precipitation to the outlet of the watershed;The concept of rill and interril erosion was utilized in conjunction with the Yahn's sediment transport equation to develop the deterministic erosion and sediment yield model component. The erosion and sediment yield model interacted with the hydrologic model to simulate the sediment yield rate from small agricultural watersheds;Parameters related to hydrology, erosion, and sediment yield models were calibrated by use of data from a small, single-cropped watershed located in Western Iowa for the year 1972. The calibrated model was then verified on that watershed by use of data for the years of 1973, 1974, and 1975, and the use of data from an adjacent watershed for the year 1972;A sensitivity analysis of the hydrologic model parameters related to infiltration and overland flow was completed. Predicted volume and rate of runoff were very sensitive to infiltration parameters. Predicted peak rate of runoff on an individual storm basis was sensitive to the hydraulic roughness coefficient. Selected parameters from the erosion and sediment yield model were also subjected to sensitivity analysis;The shortest time increment used in the model to stimulate the surface runoff and sediment yield during the periods of ranifall-runoff events was 2 minutes. Time increments of 5, 10, and 15 minutes were tested. The predicted volume of runoff was not sensitive to the time increment of less than 15 minutes; predicted rate of runoff and sediment yield were very sensitive to the duration of the time increment.