Analysis of SWAT plus model performance: A comparative study using different software and algorithms

The Soil and Water Assessment Tool plus (SWAT+) model is widely used to analyze water dynamics in hydrological processes. It improves upon the earlier SWAT version by incorporating decision tables that allow for the specification of different land use management activities and scenarios. However, accurate watershed representation requires proper calibration and validation. Among the available open-source tools for this purpose, SWAT + Toolbox and R-SWAT remain underexplored in scientific literature. This study aimed to compare and evaluate the performance of SWAT + using R-SWAT and SWAT + Toolbox by applying different calibration and validation algorithms in a rural watershed in southern Rio Grande do Sul, Brazil. The Sequential Uncertainty Fitting 2 (SUFI-2) and Dynamically Dimensioned Search (DDS) algorithms were employed to calibrate and validate both monthly and daily streamflow from 2015 to 2017, using 23 monthly and 825 daily observations. Model performance was assessed using the Nash-Sutcliffe Efficiency (NSE) and Kling-Gupta Efficiency (KGE) metrics. The results showed that SWAT + Toolbox outperformed R-SWAT, achieving better accuracy in monthly calibration (NSE = 0.69; KGE = 0.78) and daily validation (NSE = 0.74; KGE = 0.78). These findings highlight the greater precision of SWAT + Toolbox in streamflow modeling, although both tools exhibited limitations in representing baseflow. Additionally, the uncertainty analysis underscored the need for more precise input data, particularly regarding soil characterization. Future research should explore the implementation of more advanced calibration algorithms available in R-SWAT and improvements in soil characterization to enhance hydrological simulation accuracy, reduce uncertainties, and improve model reliability. Moreover, a comparative analysis between SWAT+ and its predecessor is recommended.