The purpose of this study was to simulate the hydrologic processes from a watershed using the “Soil and Water Assessment Tool” (SWAT) model approach. The model was evaluated with the purpose to simulate the streamflows in an agricultural watershed in central Thailand. The results showed that the coefficient of correlation (R2) and the Nash-Sutcliffe coefficient (ENS) values were raised above 0.7, and the deviation of runoff volumes (Dv) was also acceptably accurate. Some months of simulated flows were overestimated but most simulated flows were close to observed flow by both the graphic and the statistical approaches. Although the model was evaluated using limited data and some of the model’s algorithms for calculating flows might not be appropriate for tropical conditions like the watershed, overall prediction results were within acceptable levels for estimating monthly flows. This led to the conclusion that the SWAT model can reliably predict monthly streamflows on any other agricultural watershed in tropical climates with conditions similar to the watershed studied.