In this study, the effect of utilization of forest trees on runoff and waste water in the Kheyrud forested watershed was investigated using the SWAT model. In order to achieve this purpose, calibrating and validating the model were firstly done by putting 20 percent exploitation from each sub-basin into use, and repeated simulations of the SWAT model. The rate of monthly flows changes were obtained and then were evaluated by using of the indexes % F and f. % F index makes use of sub-region area parameter in monthly discharge analysis. The calibration and validation of the SWAT model were performed using SUFI2 algorithm in SWAT-CUP software and error evaluation criteria, respectively. The results of calibration and validation of the SWAT model showed the acceptable capability of simulation for this model. The implementation of 20 percent showed that increasing the exploitation percent of forest trees in sub basins 3, 4, and 5, was resulted in increasing the monthly discharge, while the implementation of 20 percent in sub basins 1 and 2 presented no tangible effect on the increase of the watershed’s monthly discharge. Accordingly, sub-basins situated in the middle of the Kheyrud watershed, especially sub basins 4 and 5, due to the concurrency of the sub-basin discharge at the outlet point of the catchment, accompanied with the proper physiographic conditions providing a condition for the watershed to enjoy a greater monthly discharge. These results can be useful in managing forest exploitation and selecting the most suitable utilization places.