MODFLOW is a three-dimensional finite difference groundwater model. It uses modular structure, which makes it easy to modify to adapt to specific application code. Modflow-2000 simulates stable and unsteady flow in irregular flow systems. Aquifers can be closed or unclosed, or a combination of the two, flowing under external pressure, such as flowing to wells, surface recharge, evaporation, flowing to drainage channels, and flowing through riverbed. In addition to simulating groundwater flow, some related functions are extended, such as sediment transport.