# Basin

General watershed attributes are defined in two basin input files, [**codes.bsn**](https://swatplus.gitbook.io/io-docs/introduction-1/basin-1/codes.bsn) and [**parameters.bsn**](https://swatplus.gitbook.io/io-docs/introduction-1/basin-1/parameters.bsn). These attributes control a diversity of physical processes at the watershed level. Users can use the default values set by the interfaces or change them to better reflect what is happening in a given watershed. 

{% hint style="info" %}
Even if nutrients are not being studied in a watershed, some attention must be paid to basin nutrient variables, because nutrient cycling impacts plant growth, which in turn affects the hydrologic cycle. \
Variables governing bacteria or pesticide transport need to be initialized only if these processes are being modeled in the watershed. 
{% endhint %}