Hydrologists study the distribution, movement, and quality of underground and surface water. Hydrologists are often involved in the design of irrigation systems, waste treatment plants, hydroelectric power plants, and flood warning systems. Some hydrologists work very closely with government officials to help them develop plans for water conservation. They may also work with scientists who study marine animals, and engineers who are designing a dam or reservoir.

Because hydrology is such a large field, most people choose to specialise in researching a particular water source.

Some common specialisations include:

Surface water hydrologists study water from lakes, streams, rivers, oceans, and other above-ground water sources. Their research has many uses in the real world. For example, they help forecast floods and droughts, and inform reservoir operators on how they should manage their water supply.

Groundwater hydrologists study water that's found below the earth's surface. They are normally interested in the sustainability, reliability, quality, and quantity of the groundwater. Their research may be used to help people decide where they should pump water from. It also helps people decide where they should put waste disposal facilities, so the groundwater isn't negatively impacted.

Hydrometeorologists study the relationship between water on the earth's surface and the water in the atmosphere. By studying the evaporation rates of different bodies of water, these hydrologists can help predict droughts.