MODFLOW is the USGS finite-difference computer model that simulates groundwater flow and heads in aquifers in three dimensions.
MODFLOW is a groundwater flow model made and kept up by the US Geological Survey (USGS). It is free, public domain and one of the most widely used codes for simulating how water moves through aquifers. Agencies, universities and engineers use it to answer questions such as how far a cone of depression will spread, how much a river will lose to pumping, or how recharge changes the water table. The model splits the aquifer into a grid of cells arranged in rows, columns and layers. In each cell it solves the groundwater flow equation, which combines Darcy's law with conservation of mass. The equation links the change in hydraulic head over time to flow in and out of the cell, the hydraulic conductivity and the storage of the rock or soil. The code is modular: separate packages add wells, recharge, rivers, drains, evapotranspiration and other boundary conditions. Inputs are the grid and layer elevations, hydraulic conductivity, specific storage or specific yield, starting heads, boundary conditions and stresses such as pumping rates over time. Outputs are the head in every cell at every time step and a water budget that shows flow between cells and across each boundary. For example, a town planning a new well field can build a model of the aquifer, calibrate it to observed heads in monitoring wells, then run it with the new pumping to see drawdown at nearby farm wells and the drop in baseflow to a stream. MODFLOW is well tested, well documented and supported by many free and paid graphical interfaces. Companion codes add particle tracking and solute transport. Its limits are those of the data: results depend on how well conductivity and recharge are known, and a model can match heads with more than one set of parameters. Building a sound model needs good geology and careful calibration.