"Hydrological Sciences" is a very large waste basket. You definitely will have to develop a large number of sub-sections. For instance, I am a glaciologist interested in ice in the sea. This is a subject that deals with a partially solid material that develops from sea water when the oceanographic and meteorological conditions are suitable. Not surprisingly its properties and behavior are extremely variable. This field is certainly a subset of geophysics. Whether it is a subset of hydrology is another matter. You certainly never find its existence discussed in texts on hydrology. W. F. Weeks