> 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/reservoirs/hydrology.res/mon_op.md).

# mon\_op

If 0 is input for [yr\_op](/io-docs/introduction-1/reservoirs/hydrology.res/yr_op.md) and *mon\_op*, the model assumes the reservoir is in operation at the beginning of the simulation.