Integrating manual calibration and auto-calibration of SWAT model in Muar Watershed, Johor

The Soil and Water Assessment Tool (SWAT) is a freely accessible hydrological modeling tool designed to simulate the impact of land-cover changes on hydrological processes over a continuous period. To test its performance in tropical climate conditions, the hydrological processes of the upper Muar watershed (UMW), Malaysia was modeled for land cover data in the year 1984. The SWAT model was run using 42 years of daily climate data from 1970 to 2011, with a six-year warm-up period (1970 to 1975). Manual calibration and auto-calibration using Sequential Uncertainty Fitting (SUFI-2) were coupled to optimize the Nash-Sutcliffe efficiency (NSE) and coefficient of determination (R²) value. Parameter CN2, CANMX, SOLAWC, GWREVAP and GWQMN are five of the most sensitive parameters for UMW. Meanwhile, EPCO, GWDELAY and CHN(2), found as less sensitive parameters to be adjusted. The value of NSE_{calibration-validation} = 0.71 & 0.62, and R²_{calibration-validation} = 0.74 & 0.68 indicate of satisfied results obtained. The P-factor and R-factor for calibration was 0.92 and 1.77 respectively and P-factor=0.94, R-factor = 2.04 during validation period. This study therefore suggests that manual and auto-calibration approaches are suitable to be integrated to optimize NSE and R² values. These goodness-of-fit statistics, supported by graphical representations, showed that the SWAT model could simulate a tropical watershed with reasonable accuracy.