# Hydrology

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

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