In Source, the term constituents refers to materials that are generated, transported and transformed within a catchment and affect water quality. Common examples include sediments, nutrients, salts and other dissolved solids. 

Processes that act on constituents to generate, transport and transform them can be modelled in Source. These models are broadly categorised as Catchment Water Quality models and Storage and Link Water Quality models.

Catchment water quality models are classified as:

Storage and link water quality models include: