Inflow

Inflow is the act or process of something flowing into a system from outside, or the amount that does so over a given period. It is the counterpart to outflow, which describes flow out of a system. In many disciplines, inflow is a key component of balance calculations, where the change in a system’s state is determined by the difference between inflows and outflows.

In hydrology and water resource management, inflow refers to water entering a reservoir, basin, or watershed. Common sources include precipitation, river or stream inflows, groundwater recharge, and surface runoff. Inflow data are essential for reservoir operations, flood forecasting, drought assessment, and water accounting. In groundwater studies, inflow often denotes recharge from surrounding aquifers or the land surface.

In economics and finance, inflow describes money or resources entering an entity. Examples include cash inflows from sales, investment income, loans, grants, and remittances. Inflows influence liquidity, funding availability, and financial planning, and are typically contrasted with outflows such as expenditures, debt service, and operating costs.

Other uses extend to domains such as data systems, where inflow can describe incoming data streams; in demographics, the inflow of people into a region or country; and in process engineering, where inflow denotes the input streams to a reactor or production line. In all contexts, inflow is a measure of external input contributing to the growth, capacity, or state of a system, often analyzed through mass balance or cash-flow accounting.