Simulating Surface and Groundwater Hydrological Process in Big Sunflower River Watershed By Using SWAT and Modflow Model.

The Big Sunflower River Watershed (BSRW) is a highly productive agricultural region in Mississippi with nearly 81% of the area being cropland. Over 700,000 hectares of row crops in this region was irrigated by using groundwater. As a result, groundwater level has declined > 6.5 m (20 ft) since 1970, which threaten the sustainability of irrigated agriculture in this region. Therefore conjunctive use of surface and groundwater for irrigation is needed in this region. The objective of this study was to calibrate and validate the SWAT and MODFLOW models separately for analyzing the surface and groundwater hydrological process. For daily stream flow, the SWAT model performed well during the calibration period (R² ranged from 0.46 to 0.69 and Nash-Sutcliffe efficiency varied from 0.41 to 0.62) and validation period (R² ranged from 0.64 to 0.79 and Nash-Sutcliffe efficiency varied from 0.63 to 0.77). The MODFLOW model simulated the groundwater distribution and groundwater head in our study area quite well. The MODFLOW model showed that the average change in yearly groundwater level increased, as groundwater pumping decreasing. If groundwater pumping was reduced by 25% -35%, the average yearly ground water level change was zero, which means 25% -30% of irrigation would be needed to be supplied by surface water to reduce the change in groundwater level to near zero. Coupling use of surface and ground water for irrigation is a sustainable way for water resources management in this region. Appropriate percentages of use of groundwater and surface waters were determined under current weather conditions and future climate change scenarios.