In hydrology, the inflow of a body of water is the source of the water in the body of water. It can also refer to the average volume of incoming water in unit time. It is contrasted with outflow. Outflow is the amount of water that leaves the country. Inflow is mostly used when referring to rivers, and the amount of water in units that enters the country.

All bodies of water have multiple inflows, but often, one inflow may predominate and be the largest source of water. However, in many cases, no single inflow will predominate and there will be multiple primary inflows. For a lake, the inflow may be a river or stream that literally flows into the lake. Inflow may also be, strictly speaking, not flows, but rather precipitation, like rain.

Inflow can also be used to refer to groundwater recharge.

Hydrology is the study of the movement, distribution, and quality of water on Earth and other planets, including the hydrologic cycle, water resources and environmental watershed sustainability. A practitioner of hydrology is a hydrologist, working within the fields of earth or environmental science, physical geography, geology or civil and environmental engineering.

Domains of hydrology include hydrometeorology, surface hydrology, hydrogeology, drainage basin management and water quality, where water plays the central role. Oceanography and meteorology are not included because water is only one of many important aspects within those fields.

Hydrological research can inform environmental engineering, policy and planning.

The term hydrology is from Greek: ὕδωρ, hydōr, "water"; and λόγος, logos, "study".

Hydrology has been a subject of investigation and engineering for millennia. For example, about 4000 B.C. the Nile was dammed to improve agricultural productivity of previously barren lands. Mesopotamian towns were protected from flooding with high earthen walls. Aqueducts were built by the Greeks and Ancient Romans, while the history of China shows they built irrigation and flood control works. The ancient Sinhalese used hydrology to build complex irrigation works in Sri Lanka, also known for invention of the Valve Pit which allowed construction of large reservoirs, anicuts and canals which still function.