Mapping field dependencies and web forms

Mapping field dependencies and web forms

If I have created field dependencies and need these to be populated based on the details provided in the web forms, how do I achieve this? 

As example assume I have a primary field called Department and based on the department the user selects, it then populates the required data related to the primary field Department. 

There is a one to one relationship and the user is not shown the dependency. All they are selecting is the department they belong to.