Comparison between Structured and Unstructured MODFLOW for Simulating Groundwater Flow in Three-Dimensional Multilayer Quaternary Aquifer of East Nile Delta, Egypt

Abstract

This study presents the differences and similarities between the MODFLOW-USG (Unstructured Grid version of MODFLOW) and traditional structured versions of MODFLOW 2005 in simulating a three-dimensional multilayer groundwater flow model in Quaternary aquifer east Nile delta in Egypt. A 390-borehole dataset was used to build a three-dimension stratigraphic model of the Quaternary aquifer. GIS-based conceptual model is mapping to represent the different boundary conditions such as lakes, river, drains, aquifer recharge, and discharge. The conceptual model is exported to the stratigraphic model; therefore, boundary conditions data are automatically arranged in arrays identical with the grid cells. MODFLOW 2005 and MODFLOW USG are combined with the computer-based program PEST to calibrate groundwater head distribution through the aquifer system. Groundwater levels measured in 1991 are used for the steady-state calibration and are employed then as initial conditions for the transient calibration between 1991 and 2004. The heads measured in 2015 are used for more model verification. The results show that MODFLOW-USG provides flexibility in grid design, including the capability to use nonrectangular cell shapes. Also, it can be used with simple grids and nested grids that allow for the solution to be focused in areas of interest. Also, the results show that the MODFLOW-2005 simulation takes about 20 times longer to complete the run and uses about 6 times more cells number than the MODFLOW-USG simulation.