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.