This question is an exact duplicate of:
I have a page that presents different topics (These are terms in the term store). When a user clicks on a topic the are brought to a standard search results page with a Search Box, Refinement Panel (with the topic checked off), and Search Results related to the topic chosen on the previous screen.
I would like to have the user enter a keyword search to further refine the search, but when I do the selected topic (refiner) is being lost. Only the keywords are contained in the query.