Extended Abstract
Introduction: A watershed system regulates both the quantity and quality of water within the hydrological cycle. Challenges have arisen in managing this cycle, largely due to insufficient understanding of its complexity and inadequate planning regarding the interconnections between water resource management and community development. Effective watershed management requires comprehensive and accurate information on various technical and managerial approaches. Simulating hydrological processes within a watershed is considered a promising approach for achieving optimal management. This study aims to develop and evaluate a new rainfall-runoff model for the Meymeh Watershed as a mountainous watershed located in a semi-arid region of Ilam Province, Iran.
Materials and methods:  This study was conducted using the new SWAT+ model. SWAT+ is a powerful tool for achieving watershed management objectives, offering a flexible spatial representation of basin processes and responses. It integrates a large number of parameters, utilizes the free QGIS software, and features a robust graphical interface.The data required for this research include meteorological records from the watershed and its surrounding areas, historical flow data of the Meymeh River, a Digital Elevation Model (DEM), and geological and soil maps. Meteorological data were collected from two synoptic stations near the watershed and 20 rain gauges located within and around the watershed, sourced from governmental organizations. Historical and observed daily flow data from a hydrometric station at the watershed outlet were also obtained from existing databases.Daily meteorological and hydrological data from 2010 to 2020 were used to simulate streamflow in the study area. Considering that using multiple statistical indicators can lead to mixed interpretations of model performance, in this study were employed the coefficient of determination (R²), Nash–Sutcliffe Efficiency (NSE), Mean Absolute Error (MAE), and Mean Bias Error (MBE) to evaluate the accuracy and reliability of the model.
Results and Discussion: Based on the results, NS, R² coefficients, MAE and MBE were obtained -0.38 , 0.39, 11.1 and 8.4 respectively, using non-optimized coefficients using in the initial run of the model. According to the value of the objective functions in the first run, it was found that the SWAT+ model has insufficient accuracy for the watershed runoff simulation, so the calibration operation is necessary to improve its accuracy. For calibration, ten coefficients and parameters that are effective in producing watershed runoff were determined. These parameters were entered into the model along with the allowed range of their changes (Theoretically) and were real and optimized during 2000 iterations. Following this process, the R², Nash-Sutcliffe coefficients, MAE and MBE for the calibration period (2010-2018) were obtained 0.72, 0.70, 2.97 and 0.58 respectively, and for the validation period (2019-2020), 0.78, 0.77, 7.6 and 0.38 respectively. In order to evaluate the ability of the model in simulating base and peak flows and also checking their temporal consistency with the observed data, scatter plots and time series of observed and simulated daily flow values were drawn for the calibration and validation periods. A detailed review of the drawn graphs showed that this model has correctly identified the time of the peak flows. Also, the daily fluctuations of the river flow are correctly modeled. From a graphical point of view, the comparison of the time series plots during the validation period shows that the SWAT+ model estimated the peak and base flows close to the actual values.
Conclusion: The results of this study showed that SWAT+ has a good ability to simulate of daily runoff in The Meymeh river watershed. It can also  be applied to simulate runoff under different