Requiring User to Provide Query Information

The WFS I am working on will serve weather data (max temperature, cloud cover amount, etc.) at a given point (latitude/longitude) and at a given time. To do this I need the user to specify which weather element they want, what latitude/longitude they need the data for, and at what time they want to get the data for. How do you communicate in the capabilities document that the user [b]must[/b] provide the weather element, location, and time for which they want data? I considered using the Filter_Capabilities element, but that seems to indicate what filters are possible, not that a filter must be used.