Hydrology is an extremely important field of study, dealing with one of the most valuable resources on Earth: water. All aspects of the Earth’s available water are studied by experts from many disciplines, from geologists to engineers, to obtain the information needed to manage this vital resource. Hydrologists rely on their understanding of how water interacts with its environment, including how it moves from the Earth’s surface, to the atmosphere, and then back to Earth. This never-ending movement is called the hydrologic cycle, or the water cycle.

Water takes on various forms in the environment in response to changes in temperature and other influences. Water from the surface of oceans and other bodies of water is warmed by the sun and evaporates as water vapor. As this moist air rises high into the atmosphere, it cools and condenses into clouds. Moisture in the clouds then returns to the Earth’s surface as precipitation. Once it reaches the ground, the water is absorbed, and it becomes groundwater. Groundwater that is not absorbed will return to creeks, rivers, streams, and eventually to the oceans. The cycle repeats itself as the surface of bodies of water once again evaporates. Moisture captured by plants can also return to the atmosphere through a process called transpiration.

The field of hydrology consists not only of studying the natural distribution and movement of water, it is also concerned with the impact of human activities on water quality and with problems in water management. People use water for many purposes. In their homes, people use water for drinking, cooking, cleaning, and bathing. Many industries have a great need for water. In agriculture, water is used for the irrigation of farmland and for livestock. Water in many dams is used to produce hydroelectric power. The list of human uses for water is virtually endless.

Hydrologists, such as National Geographic Explorer Manase Elisa, play a critical role in determining the impact of human activities on our available water. Elisa conducted the first extensive study of the Katuma River-Lake Rukwa ecosystem in the African nation of Tanzania. Irrigation in the Katuma River region has led to environmental degradation downstream. Hydrologists strive to improve water quality and increase our access to water so that we can continue to make use of it in all the ways that are necessary to our lives.