# 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. --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://swatplus.gitbook.io/io-docs/introduction-1/hydrology.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.