ParFlow

ParFlow simulates water fluxes of surface, soil and groundwater for example infiltration of rainwater and evaporation and transpiration, including anthropogenic influences e.g. extensive irrigation from a few days to seasonal modeling developed by Forschungszentrum Jülich and several more institutions. Thus, it simulates feedback processes and components of the hydrologic cycle that are also subject to human-induced changes.

The software is open-source under GNU LPGL license agreement.

More information, features, publications and applications about ParFlow here.

If you want to get started, the software is available on GitHub. The documentation is filed on ReadtheDocs, which includes a "Getting started" section.
If you have more questions there is also a mailing list and blog.