The USGS has been a leader in the development of hydrologic and geochemical simulation models since the 1960's. USGS models are widely used to predict responses of hydrologic systems to changing stresses, such as increases in precipitation or ground-water pumping rates, as well as to predict the fate and movement of solutes and contaminants in water. Today, USGS models are available for free on the internet.

See http://h2o.usgs.gov/software/ to obtain software, which has been grouped into the following categories: Geochemical, Ground Water, Surface Water, Water Quality, General.

See http://water.usgs.gov/nrp/gwsoftware/ to obtain some of the newest USGS computer software, designed to model various ground-water and related hydrologic and geochemical processes and to help estimate related parameters.

See http://wwwbrr.cr.usgs.gov/projects/SW_precip_runoff/mms/index.html to download the Modular Modeling System (MMS) software, a set of modeling tools that provide a framework that can incorporate a variety of models and thus create an "optimal" model to be used, for example, in watershed management.

Use the USGS Surface-water quality and flow Modeling Interest Group (SMIG), to network with other surface-water modelers, keep abreast of current surface-water modeling activities in the USGS, discuss new modeling techniques, algorithms, interfaces, and learn about new models.

Software and related material (data and documentation) are made available by the U.S. Geological Survey (USGS) to be used in the public interest and the advancement of science. You may, without any fee or cost, use, copy, modify, or distribute the software, and any derivative works thereof, and its supporting documentation, subject to the USGS software User's Rights Notice.