Linked from
The 36 pages that link to Hydrology, each with the reason it gives.
PrecipitationNarrower topic: Precipitation supplies water to rivers, lakes, soils, and aquifers.
PeatlandNarrower topic: Water inputs and drainage govern peatland saturation and persistence.
Continental DivideNarrower topic: Hydrologists use divides to define and compare drainage areas.
Soil physicsNarrower topic: Soil processes form a key part of terrestrial water movement.