Source : Webster's Revised Unabridged Dictionary (1913)

Hydrology \Hy*drol"o*gy\, n. [Hydro-, 1 + -logy: cf. F.
   hydrologie.]
   The science of water, its properties, phenomena, and
   distribution over the earth's surface.

Source : WordNet®

hydrology
     n : the branch of geology that studies water on the earth and in
         the atmosphere: its distribution and uses and
         conservation