Fully integrated numerical simulation of surface water-groundwater interactions using SWAT-MODFLOW with an improved calibration tool

•Efficient method to determine the most important SWAT-MODFLOW parameters when simulating surface water and groundwater.•Curve Number (CN) and GW hydraulic conductivity (K) have the most significant impact on runoff and GW level simulation.•Demonstration that properly calibrating both surface water and groundwater components of the water cycle is critical. Long-term sustainable water resources management requires comprehensive assessment of all water cycle components. The combined calibration of surface water (SW) and groundwater (GW) parameters is one of the biggest challenges in simulating SW-GW interactions. SWAT-MODFLOW has improved our ability to simulate SW and GW interactions, however there remain challenges associated with (i) understanding what parameters most influence model outputs and (ii) properly calibrating the GW related components of the water cycle (as represented within SWAT-MODFLOW). In this paper, we applied SWAT-MODFLOW to the 1452 km2 Shiraz catchment, located in southwestern Iran, for the period 2003–2019. A comprehensive sensitivity analysis (SA) of both SW and GW parameters was conducted to determine what controls runoff and GW level in the study catchment. The SA reveals that the Curve Number (CN2) and GW hydraulic conductivity (K) parameters have the most significant impact on runoff and GW level simulation. This work also presents a “GW Calibrator” (GWCal) tool along with a demonstration of how GWCal improves the performance of SWAT-MODFLOW in the Shiraz catchment as a result of the insights gained from the SA.