International Academy of Ecology and Environmental Sciences Proceedings of the International Academy of Ecology and Environmental Sciences 2220-8860 5 2 2015 06 1 Application of SWAT model for assessing effect on main functions of watershed ecosystem in Headwater, Thailand 57-69 EN W. Sudjarit Somnimirt Pukngam Nipon Tangtham http://www.iaees.org/publications/journals/piaees/articles/2015-5(2)/SWAT-model-for-assessing-effect-of-watershed.pdf> The Soil and Water Assessment Tool (SWAT) is a well prediction accuracy of agricultural watershed ecosystem depends on how well model input spatial parameters describe the characteristics of watershed. The aim of this study was to assess the effects on watershed ecosystem main functions in terms of water and sediment yield. It was calibrated and validated for streamflow in the watershed to evaluate alternative management scenarios and estimate their effects on watershed functions. The goodness of the calibration results was assessed by the coefficient of determination (R2). Results indicated that the average annual rainfall and streamflow estimations were quite satisfactory. On a daily scale R2 was about 0.69 and a monthly scale was 0.97 which can be considered as acceptable. However, using for the case study of an intensive agricultural watershed ecosystem, it was shown that model versions are able to appropriately reproduce the water balance, nutrients balance, carbon balance, and energy balance. Crop yield, total streamflow and total suspended sediment (TSS) losses calibration were performed using field survey information and data during 2008-2012. This study showed that SWAT model was able to apply for simulating and assessing streamflow, sediment, and nutrients successfully and can be used to study the effects of land use practices on water balance, nutrient balance, carbon balance and energy balance in the small scale of sub-watershed ecosystem as well.