Complex sentence: One of the well-known program in modeling groundwater flow is the MODFLOW developed by the United States Geological Survey, it is a free and open-source program that uses finite difference method as the framework to model groundwater conditions.
Split into simpler sentences: One of the well-known programs in modeling groundwater flow is MODFLOW developed by the United States Geological Survey. It is a free and open-source program that uses the finite difference method as the framework to model groundwater conditions.