Hydrologic models have been used to assess water quality performance of complex watersheds and river
basins for managing water resources systems. Hydrologic models can provide essential information to policy
makers for making decisions on sustainable management system of water resources within watersheds. A study
was conducted on the application of a watershed scale simulation model, Soil and Water Assessment Tool
(SWAT), for the Chi River Subbasin II located in northeastern Thailand. Calibration and validation of the SWAT
output were performed by comparing predicted stream flows with corresponding in-stream measurements from
four gaging stations within the watershed for four years (2000-2003). Statistical comparisons between the
simulated results and the observed data for the calibration year gave a reasonable agreement for both monthly
coefficient of determination (r2) and Nash-Sutcliffe Coefficient (E) within ranges of 0.77-0.88 and 0.55-0.79,
whereas the validation results showed lower values of r2 and E ranging from 0.23-0.77 and -7.98-0.66. Overall,
the SWAT model has the capability to predict stream flows within the Chi River Subbasin II in northeast
Thailand.