The aqueous layer of the tear film is produced by the lacrimal glands (in the upper eyelids). This is the thick middle layer of the tear film. The mucin layer underneath helps it adhere to the epithelium and the lipid layer on top protects it from rapid evaporation.
A deficiency of aqueous tears is one of the conditions that can cause dry eye syndrome. Aqueous tear deficiency can be measured with tests such as the Schirmer lacrimation test. A severe aqueous deficiency will be apparent under slit-lamp examination.
True "primary" aqueous deficiency is much less common than dryness due to meibomian gland dysfunction (lipid deficiency). Typical causes would include auto-immune disease (e.g. Sjogrens Syndrome) or dennervation as a result of LASIK eye surgery.