The Collaborative International Dictionary of English v.0.48:

Hydrometric \Hy`dro*met"ric\, Hydrometrical \Hy`dro*met"ric*al\,
   a. [Cf. F. hydrom[`e]trique.]
   1. Of or pertaining to an hydrometer, or to the determination
      of the specific gravity of fluids.
      [1913 Webster]

   2. Of or pertaining to measurement of the velocity,
      discharge, etc., of running water.
      [1913 Webster]

   3. Made by means of an hydrometer; as, hydrometric
      observations.
      [1913 Webster]

   Hydrometric pendulum, a species of hydrometer consisting of
      a hollow ball of ivory or metal suspended by a treated
      from the center of a graduated quadrant, and held in a
      stream to measure the velocity of the water by the
      inclination given to the thread; a kind of current gauge.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hydrometric
    adj 1: of or relating to hydrometry [syn: hydrometric,
           gravimetric]