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.