Discharge (hydrology)

In hydrology, discharge is the volumetric flow rate of water that is transported through a given cross-sectional area.[1] It includes any suspended solids (e.g. sediment), dissolved chemicals (e.g. CaCO3(aq)), or biologic material (e.g. diatoms) in addition to the water itself.

Synonyms vary by discipline. 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.

Theory and calculation

GH Dury and MJ Bradshaw are two hydrologists who devised the 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 stream 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.[2] For example, the average discharge of the Rhine river in Europe is 2,200 cubic metres per second (78,000 cu ft/s) or 190,000,000 cubic metres (150,000 acre⋅ft) 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:


  • is the discharge ([L3T−1]; m3/s or ft3/s)
  • is the cross-sectional area of the portion of the channel occupied by the flow ([L2]; m2 or ft2)
  • is the average flow velocity ([LT−1]; m/s or ft/s)

Catchment discharge

The catchment of a river above a certain location is determined by the surface area of all land which drains toward the river from above that point. The river's discharge at that location depends on the rainfall on the catchment or drainage area and the inflow or outflow of groundwater to or from the area, stream modifications such as dams and irrigation diversions, as well as evaporation and evapotranspiration from the area's land and plant surfaces. In storm hydrology, an important consideration is the stream's discharge hydrograph, a record of how the discharge varies over time after a precipitation event. The stream rises to a peak flow after each precipitation event, then falls in a slow recession. Because the peak flow also corresponds to the maximum water level reached during the event, it is of interest in flood studies. Analysis of the relationship between precipitation intensity and duration and the response of the stream discharge are aided by the concept of the unit hydrograph, which represents the response of stream discharge over time to the application of a hypothetical "unit" amount and duration of rainfall (e.g., half an inch over one hour). The amount of precipitation correlates to the volume of water (depending on the area of the catchment) that subsequently flows out of the river. Using the unit hydrograph method, actual historical rainfalls can be modeled mathematically to confirm characteristics of historical floods, and hypothetical "design storms" can be created for comparison to observed stream responses.

The relationship between the discharge in the stream at a given cross-section and the level of the stream is described by a rating curve. Average velocities and the cross-sectional area of the stream are measured for a given stream level. The velocity and the area give the discharge for that level. After measurements are made for several different levels, a rating table or rating curve may be developed. Once rated, the discharge in the stream may be determined by measuring the level, and determining the corresponding discharge from the rating curve. If a continuous level-recording device is located at a rated cross-section, the stream's discharge may be continuously determined.

Larger flows (higher discharges) can transport more sediment and larger particles downstream than smaller flows due to their greater force. Larger flows can also erode stream banks and damage public infrastructure.

See also


  1. ^ Buchanan, T.J. and Somers, W.P., 1969, Discharge Measurements at Gaging Stations: U.S. Geological Survey Techniques of Water-Resources Investigations, Book 3, Chapter A8, p. 1.
  2. ^ Dunne, T., and Leopold, L.B., 1978, Water in Environmental Planning: San Francisco, Calif., W.H. Freeman, pp. 257–258.

External links

  • "Chapter 14: Stage-Discharge Relationships" (PDF). USDA NRCS National Engineering Handbook. Part 630: Hydrology. USDA. April 2012.
  • USDA NRCS National Engineering Handbook. Part 630: Hydrology. USDA. May 2012.
Chalk stream

Chalk streams are streams that flow through chalk hills towards the sea. They are typically wide and shallow, and due to the filtering effect of the chalk their waters are alkaline and very clear. Chalk streams are popular with fly fishermen who fish for trout on these rivers.

Current (stream)

A current, in a river or stream, is the flow of water influenced by gravity as the water moves downhill to reduce its potential energy. The current varies spatially as well as temporally within the stream, dependent upon the flow volume of water, stream gradient, and channel geometry. In tidal zones, the current in rivers and streams may reverse on the flood tide before resuming on the ebb tide.


Discharge in the context of a flow may refer to:

Electric discharge:

Discharger, an electrical device that releases stored energy

Battery discharging

Static discharger, a device used on airplanes to maintain use of electrical equipment

Electrostatic discharge, sudden and momentary electric current flows between two objects

Dielectric barrier discharge, the electrical discharge between two electrodes separated by an insulating dielectric barrier

Corona discharge, a type of electric current

Direct-current discharge, a plasma

Gas-discharge lamp, a light bulb that includes a discharge gas

Partial discharge, a temporary breakdown of electrical insulation

Discharge (hydrology), the amount of water flowing through the channel

Groundwater discharge, the volumetric flow rate of groundwater through an aquifer

Effluent released into a river or sea

Discharging method (discrete mathematics) is a proof technique in discrete mathematics

Discharge in the sense of flow of fluids from certain parts of the body:

Menstruation or other vaginal discharge

Nipple discharge, the release of fluid from the nipples of the breasts

Mucopurulent discharge, the emission or secretion of fluid containing mucus and pusDischarge in the context to expel or to "let go" may refer to:

Military discharge, when a member of the armed forces is released from service

Termination of employment, the end of an employee's duration with an employer

Patient discharge, the formal ending of inpatient care

Discharge (sentence), a criminal sentence where no punishment is imposed

The act of firing a gunDischarge in music may refer to:

Discharge (band), British hardcore punk band

Discharge (album), a self-titled album by Discharge released in 2002

"Discharge", a song by Anthrax from Persistence of TimeOther uses of discharge include:

Bankruptcy discharge, the injunction that bars acts to enforce certain debts

In co-counselling, the ways in which pent-up emotional hurt can be released, e.g. via crying, laughter, etc.

Discharge petition, the process of bringing a bill out of committee to the floor for a vote without the cooperation of leadership

Flow rate

Flow rate may refer to:

Flow measurement, a quantification of bulk fluid movement

Mass flow rate, the mass of a substance which passes per unit of time

Volumetric flow rate, the volume of fluid which passes per unit time

Discharge (hydrology), volume rate of water flow that is transported through a given cross-sectional area, such as a river

Groundwater discharge

Groundwater discharge is the volumetric flow rate of groundwater through an aquifer.

Total groundwater discharge, as reported through a specified area, is similarly expressed as:


Q is the total groundwater discharge ([L3·T−1]; m3/s),
K is the hydraulic conductivity of the aquifer ([L·T−1]; m/s),
dh/dl is the hydraulic gradient ([L·L−1]; unitless), and
A is the area which the groundwater is flowing through ([L2]; m2)

For example, this can be used to determine the flow rate of water flowing along a plane with known geometry.

Infiltration (hydrology)

Infiltration is the process by which water on the ground surface enters the soil. It is commonly used in both hydrology and soil sciences. The infiltration capacity is defined as the maximum rate of infiltration. It is most often measured in meters per day but can also be measured in other units of distance over time if necessary. The infiltration capacity decreases as the soil moisture content of soils surface layers increases. If the precipitation rate exceeds the infiltration rate, runoff will usually occur unless there is some physical barrier.

Infiltrometers, permeameters and rainfall simulators are all devices that can be used to measure infiltration rates.Infiltration is caused by multiple factors including; gravity, capillary forces, adsorption and osmosis. Many soil characteristics can also play a role in determining the rate at which infiltration occurs.

River ecosystem

River ecosystems are flowing waters that drain the landscape, and include the biotic (living) interactions amongst plants, animals and micro-organisms, as well as abiotic (nonliving) physical and chemical interactions of its many parts. River ecosystems are part of larger watershed networks or catchments, where smaller headwater streams drain into mid-size streams, which progressively drain into larger river networks.

River ecosystems are prime examples of lotic ecosystems. Lotic refers to flowing water, from the Latin lotus, meaning washed. Lotic waters range from springs only a few centimeters wide to major rivers kilometers in width. Much of this article applies to lotic ecosystems in general, including related lotic systems such as streams and springs. Lotic ecosystems can be contrasted with lentic ecosystems, which involve relatively still terrestrial waters such as lakes, ponds, and wetlands. Together, these two ecosystems form the more general study area of freshwater or aquatic ecology.

The following unifying characteristics make the ecology of running waters unique among aquatic habitats.

Flow is unidirectional.

There is a state of continuous physical change.

There is a high degree of spatial and temporal heterogeneity at all scales (microhabitats).

Variability between lotic systems is quite high.

The biota is specialized to live with flow conditions.


SaltMod is computer program for the prediction of the salinity of soil moisture, groundwater and drainage water, the depth of the watertable, and the drain discharge (hydrology) in irrigated agricultural lands, using different (geo)hydrologic conditions, varying water management options, including the use of ground water for irrigation, and several cropping rotation schedules.

The water management options include irrigation, drainage, and the use of subsurface drainage water from pipe drains, ditches or wells for irrigation.

Stream gradient

Stream gradient is the grade measured by the ratio of drop in elevation of a stream per unit horizontal distance, usually expressed as meters per kilometer or feet per mile.


Streamflow, or channel runoff, is the flow of water in streams, rivers, and other channels, and is a major element of the water cycle. It is one component of the runoff of water from the land to waterbodies, the other component being surface runoff. Water flowing in channels comes from surface runoff from adjacent hillslopes, from groundwater flow out of the ground, and from water discharged from pipes. The discharge of water flowing in a channel is measured using stream gauges or can be estimated by the Manning equation. The record of flow over time is called a hydrograph. Flooding occurs when the volume of water exceeds the capacity of the channel.

Volumetric flow rate

In physics and engineering, in particular fluid dynamics and hydrometry, the volumetric flow rate (also known as volume flow rate, rate of fluid flow or volume velocity) is the volume of fluid which passes per unit time; usually represented by the symbol Q (sometimes V̇). The SI unit is m3/s (cubic metres per second). Another unit used is sccm (standard cubic centimeters per minute).

In US customary units and imperial units, volumetric flow rate is often expressed as ft3/s (cubic feet per second) or gallons per minute (either US or imperial definitions).

Volumetric flow rate should not be confused with volumetric flux, as defined by Darcy's law and represented by the symbol q, with units of m3/(m2·s), that is, m·s−1. The integration of a flux over an area gives the volumetric flow rate.

Hydroelectricity generation
Hydroelectricity equipment


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