Hydrology today denotes a field of science that covers many more branches of science than it did earlier. Till 1906, hydrology referred to the study of underground water only, as distinct from the present usage that covers both groundwater as well as surface water. In a much broader sense, hydrology refers to the study of water.
The term ‘hydrology’ is derived from two Greek words hydor and logas meaning ‘water’ and ‘science’, respectively. So, in simple terms, hydrology is a science related to water.
The Merriam-Webster ...