This paper describes a subroutine for estimation of daily and average annual runoff in a computer model for microcatchment design and prediction of rain-fed grape yield in the Bajgah area,Farsprovince, Islamic Republic of Iran. In this subroutine, it is assumed that all abstractions arise from infiltration, and a method for determining ponding time and infiltration by using recording rain gage data and soil physical properties was developed based on the Green-Ampt infiltration equation. This subroutine was then incorporated into a previously developed model to design the microcatchment area and grape yield prediction. The developed subroutine resulted in a daily microcatchment runoff coefficient of 0.0737 in the study area which is similar to the measured value of 0.080. The daily threshold rainfall to produce daily runoff estimated by the developed subroutine was 6.5 mm where its measured value was 4.6 mm. The developed subroutine resulted in a microcatchment average annual runoff coefficient of 0.0894 in the study area, which is similar to the measured value of 0.0875. The annual threshold rainfall to produce annual runoff was estimated by the developed subroutine, and was 158.8 mm where its measured value was 106.5 mm. The estimated relationship between annual runoff and rainfall was used in the model and estimated the microcatchment area and grape yield properly. In general, it is indicated that the developed subroutine is able to determine the daily and annual runoff-rainfall relationship to be used in the model for the design of the micricatchment area and prediction of grape yield in the study area.