Introduction to MODFLOW

MODFLOW is a computer program published by the United States Geological Survey, which simulates the real groundwater table through the following steps:

• For a location of interest, such as a watershed or well-protection area, a 3-D grid is superimposed over the subsurface.
• Boundary conditions are imposed on the gird. Boundary conditions are the pathways through which water enters and exits the grid. These might include rivers, rainfall, lakes, or adjacent aquifers.
• MODFLOW determines groundwater head in each cell of the grid based on the boundary conditions.

The term groundwater head refers to the groundwater elevation in the uppermost layer of the grid. In the lower layers, groundwater head corresponds to the water pressure within that cell. Once the groundwater head throughout the grid is known, it can be used to predict groundwater flow direction and velocity.

MODFLOW determines the groundwater head of each cell by solving a set simultaneous equations, the partial differential equation of groundwater flow, one written for each cell. The equations are solved iteratively until the groundwater heads becomes stable across iterations.

GroundwaterGo uses the MODFLOW-2005 version of the program. To learn more about MODFLOW and to download the program and user’s manuals, visit the USGS MODFLOW page.