> For the complete documentation index, see [llms.txt](https://swatplus.gitbook.io/io-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://swatplus.gitbook.io/io-docs/introduction-1/aquifers/aquifer.aqu/id-aquifer.aqu.md).

# id (aquifer.aqu)

The ID of the aquifer is a primary key referenced by [aqu](/io-docs/introduction-1/connectivity/hru.con/hru.md) in [**aquifer.con**](/io-docs/introduction-1/connectivity/hru.con.md).