Discharge (hydrology)

Discharge (hydrology)

In hydrology, discharge is the volume rate of water flow, including any suspended solids (i.e. sediment), dissolved chemical species (i.e. CaCO_3(aq)) and/or biologic material (i.e. diatoms), which is transported through a given cross-sectional area. Frequently, other terms synonymous with discharge are used to describe the volumetric flow rate of water and are typically discipline dependent. For example, a fluvial hydrologist studying natural river systems may define discharge as streamflow, whereas an engineer operating a reservoir system might define discharge as outflow, which is contrasted with inflow.

GH Dury and MJ Bradshaw are two hydrologists who devised models showing the relationship between discharge and other variables in a river. The Bradshaw model described how pebble size and other variables change from source to mouth; while Dury considered the relationships between discharge and variables such as slope and friction.

The units that are typically used to express discharge include m³/s (cubic meters per second), ft³/s (cubic feet per second or cfs) and/or acre-feet per day. For example, the average discharge of the Rhine river in Europe is 2,200 cubic metres per second (78,000 cu ft/s) or 154,000 acre feet (190,000,000 m3) per day.

A commonly applied methodology for measuring, and estimating, the discharge of a river is based on a simplified form of the continuity equation. The equation implies that for any incompressible fluid, such as liquid water, the discharge (Q) is equal to the product of the stream's cross-sectional area (A) and its mean velocity, and is written as:

where

• is the discharge (m3/s or ft3/s)
• is the cross-sectional area of the portion of the channel occupied by the flow (m2 or ft2)
• is the average flow velocity (m/s or ft/s)