One of the most essential groundwater model components is accurate information about the recharge values within the input data, often introduced to a groundwater model as a percentage of rainfall on aquifers. Recharge values are influenced by many temporal and spatial factors. This paper suggests the use of a SWAT model for surface water simulation and the estimation of recharge rates. In this research, sensitivity analysis, calibration, validation and uncertainty analysis of results were performed by SWAT-CUP software. Due to the semi-distributed features of SWAT and the difficulty of calculating groundwater distributed parameters, recharge values estimated by SWAT were used in a MODFLOW model for groundwater simulation at steady and unsteady states. This method was applied in the Firoozabad basin, which is one of the most suitable agricultural basins for modeling surface water and groundwater in Iran. After MODFLOW model calibration, hydrodynamic coefficients of the aquifer were determined and the sensitivity of the model was checked for hydraulic conductivity and discharge rate of wells. In order to prove confidence, the model was validated. SWAT and MODFLOW models were successfully tested and the results of the combination of the two models were found to be acceptable.

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Recharge is considered a key parameter in groundwater models for sustainable management of aquifers, which is influenced by factors such as land use, soil, weather, etc. The present study was conducted to couple WetSpass-M and MODFLOW models for evaluating Neyshabour aquifer condition in steady and transient states. To this aim, the simulated recharge by the WetSpass-M model was applied as an input of MODFLOW to assess the groundwater balance. The hydrodynamic coefficients were determined by calibrating the model, evaluating and the model sensitivity to the hydraulic conductivity coefficient, specific yield (Sy), and recharge. The results indicated that the annual average of surface runoff, actual evapotranspiration, interception, and recharge during 1991–2017 equaled 18, 36, 7.6, and 42.6% of the average annual precipitation in the basin, respectively, with the simulated water balance error 4.2%. The average annual recharge of the basin varies between 0-257.41 mm with an average of 105.25 mm/y. Accordingly, the maximum and minimum average monthly recharge occurs during March and July, respectively. The appropriate matching of the simulated and observed water levels and obtaining the suitable values of RMSE, R2, ME, and MAE evaluation criteria in steady and transient states indicate the adequate accuracy of the WetSpass-M model in estimating the recharge and success of the couple two models. Based on the simulated groundwater balance, the aquifer faces a deficit of 421.3 MCM per year and 97.41 cm in the annual groundwater level. The model displayed more sensitivity to the hydraulic conductivity coefficient compared to other parameters.

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