> 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/hydrology.md).

# Hydrology

There are four files containing the basic topographical and hydrological properties of HRUs: 

* [topography.hyd](/io-docs/introduction-1/hydrology/topography.hyd.md) defines the topographic characteristics of the HRUs and routing units,
* [hydrology.hyd](/io-docs/introduction-1/hydrology/hydrology.hyd.md) defines the hydrological characteristics of the HRUs,
* [field.fld](/io-docs/introduction-1/hydrology/field.fld.md) specifies the properties of representative fields,
* [snow.sno](/io-docs/introduction-1/hydrology/snow.sno.md) controls the simulation of snowfall and snowmelt processes.