Hydrology

The water cycle, groundwater, aquifers, and flood frequency

What is hydrology?

Hydrology is the study of water on and beneath Earth's land surface — rivers, lakes, groundwater, and the constant movement of water between them. Unlike oceanography, which studies the ocean itself, hydrology focuses on fresh water and how it moves through the landscape.

The water cycle

Water is constantly moving between the atmosphere, the land, and the ocean in what's called the water cycle. Water evaporates from oceans, lakes, and rivers into the atmosphere, forms clouds, and falls back down as precipitation. Once on land, that water either flows across the surface into streams and rivers (runoff), soaks into the ground, or evaporates again, eventually making its way back to the ocean to repeat the cycle. This continuous cycle is the ultimate source of all the fresh water available on land.

Rivers and watersheds

A watershed is the entire area of land that drains into a particular river or stream — all the rain and snowmelt falling anywhere within a watershed's boundaries eventually flows toward the same waterway. Watersheds range enormously in size, from a small local creek's drainage area to the vast watershed of a major river system draining an entire continent's interior.

Groundwater

Not all water that soaks into the ground stays near the surface — much of it continues downward until it reaches saturated rock or soil, forming an underground reservoir called an aquifer. Wells drilled into an aquifer can draw up this groundwater for drinking and irrigation, and aquifers supply a substantial share of the fresh water used by people worldwide, especially in areas without abundant surface rivers or lakes.

Why hydrology matters

Hydrology guides decisions about water supply for drinking, farming, and industry, helps predict and manage flooding, supports the design of dams and reservoirs, and is essential for understanding and protecting groundwater resources that, once contaminated or depleted, can take a very long time to recover.