NIRAMS II is a simple, spatially distributed nitrate leaching model designed to estimate nitrate loadings to groundwater from the base of the soil profile. The model was developed by the James Hutton Institute and has been used to support the Scottish Government's four-yearly review of the EU Nitrates Directive.
If you're installing from scratch on Windows then I thoroughly recommend the WinPython installation.
Background to the model's development, its uses and some basic documentation are provided here.
A report detailing how NIRAMS II was used to support the 2013 review of the Nitrates Directive in Scotland is available here.