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Simulation of steady-state groundwater flow and evaluation of groundwater level fluctuations in the Mississippi Delta using GMS-MODFLOW
Proceedings of the 2023 Mississippi Water Resources Conference

Year: 2023 Authors: Nekooei M., Paz J.

In recent years, significant water withdrawal from the Mississippi River Valley Alluvial Aquifer (MRVA) for irrigated agriculture in the Mississippi Delta region have caused considerable declines in aquifer water levels, to the extent that withdrawals have outstripped the recharge. Therefore, water managers have a major concern about the ability of aquifers to meet increasing water demand in the Delta. In this research, the MODFLOW groundwater flow model with GMS software was used to compare the simulated daily groundwater levels with the water-level measurements in the period of the growing season from April through September between 2016 to 2021. The daily groundwater data for 11 observation wells were collected from USGS for the period, along with a few available pumping data from the irrigated wells in the Delta. In addition, the data for areal recharge, rivers, adjacent aquifers, boundaries, and the characteristics of the aquifer were considered as the other GMS packages to construct the model. The discretized was established by using a single layer rectangular-grid and oriented north-south with 1-kilometer square cells to represent the alluvial aquifer. The part of the grid covering the Delta area has 76,039 active cells. The model-generated April to September water levels for each year of the simulation were compared to the corresponding measured water levels. The average root-mean-square (RMS) error determined for the calibrated model was 1.4 meter. Assessing the daily data, a declining trend in groundwater levels was observed during the growing season throughout the studied area. However, the aquifer experienced a more reduction in the water table in 2016 and 2017 in comparison with the other years. In addition, the counties of Sunflower, Leflore, and Washington experienced more fluctuations in the water table due to the excessive withdrawals. Overall, the model results indicated that the simulated hydrographs in all the wells are reasonable representations of the measured water levels. A long-term modeling scenario that assumes the same 2021 pumping rate will likely result in the depletion of grid cells located in the three counties. Based on the model results, the net recharge from the Mississippi River and from the adjacent aquifers was small.

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