The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hydrology
    n 1: the branch of geology that studies water on the earth and
         in the atmosphere: its distribution and uses and
         conservation